1880 - laurie - the embryology of a scorpion (euscorpius italicus)

8/7/2019 1880 - Laurie - The Embryology of a Scorpion (Euscorpius Italicus)

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THE EMBRYOLOGY OF A SCORPION. 1 0 5

The Embryology of a Scorpion (Euscorpiusitalicus).By

Malcolm Laurie, B .Sc ,Falconer Fellow of Edinburgh University.

With Plates XIIIXVIII.

SINCE 1870 there has been no detailed work on the de-velopment of the Scorpion. As it seemed likely that withmodern methods of section-cutting and the great advancewhich has been made of late years in the field of embryology,a renewed exam ination might yield intere sting results, Ihave, at Professor Lankester's suggestion, examined and cutsections of a l arge number of embryos of E u s c o r p i u s i ta l i c u spreserved for him by th e Zoological S tatio n at Na ples. I havealso examined- a number of embryos of Scorp io (Buthus)f u lv ip e s preserved and sent over from Ma dras by ProfessorBo urne . The se, however, chiefly owing to the small amountof food-yolk, show such a great difference from E . i t a l i c u sin their mode of development that it seems better to postponethe description of them to a future paper.The Scorpion is interesting not only as being the lowest, and,as far as we know, the oldest type of air-breathing Arachnid,but also as being exceptional among Arthro pods in that thewhole development takes place within the body of the femalein the ovarian tube s. The only other instance s of this withwhich I am acquainted are Phrynus, which is also viviparous,

VOL. XX XI ,. PART II . NEW SER. H


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106 MALCOLM LAURIE.and S ph oe ro g yn a v e n tr ic o s a , one of the A.carina in whichthe young are born sexually mature.I may fitly here express my thanks to Professor Ray Lan-kester not only for the suggestion that I should work at thisinteresting subject, and for the generous way in which he hasprovided me with material, but even more for his continualand invaluable assistance and advice while the work has beenin progress.

HISTORICAL INTRODUCTION.Johannes Miiller1 gave a short description, with five or sixfigures, of the development of But hu s. Owing to its brevityand the absence of any attempt to ascertain the internalarrangement, his paper is of little value except from an histori-cal point of view.Duvernoy2 gives also only a few figures of Buthus andof ano ther form, probably Eus corp ius. He describes at someleng th a cord (baguette) which he says passes from the appendixof the follicle in Buthus to the mouth of the embryo, and which

Miiller had compared to an umbilical cord. I hope to be ablein a future paper to give a detailed account of this and othercurious points in the development of Bu thu s. The chief valueof Du verno y's paper was tha t he reconciled the contradicto rydescrip tions of the ovary which had been given by Miiller andRathke.3 While doing this he makes a rather serious mistakein describing the ovum of Buth us as occupying the whole ofth e diverticulum of th e ovarian tub e, instead of only a smallspace at the top.The n ext wr iter on th is sub ject is Leon Dufonr,* who gives

1 Joh. Muiler, " Beit. z. Anat. des Skorpions," ' Meckel's Arch. f. Anat.u. Phys.,' Bd. xiii, 1828.- Duvernoy, " Fragments sur les organes de la ge'ne'ration de divers Aui-maux," 'Mem. de 1'Acad. des Soi. de l'lnstitut,' t. xxiii.3 Rathke, "Z ur A lorphologie," ' Reisbemerkungen aus Taurien,' 1837,Riga, 4to.* Dufour, " Hist. Anat. et Phy s. de Scorpions," Mem. Presented a 1'Acad.des Sciences,' t. xiv, 1856.


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THE EMBRYOLOGY OF A SCORPION. 107an elaborate description with numerous figures of the anatomyof the adult . His description of th e embryo is, however, verybrief and his figures unsatisfactory.

Elias Metschnikoff1 is the only writer who has treated of thedevelopment of the Scorpion with any degree of fulness. Hegives a detailed account of the whole development, and hispaper, which deals chiefly with the surface views and opticalsections, contains a large amount of accurate and laboriousobservation. I t is the classic on this subject, and up to 1886no attempt was made to add to it or supersede it.

In 1886 Kowalevsky and Schulgin3 published a short accountof the development of A n d r o c t o n u s o r n a t u s . Unfortu-nately their paper has no figures, which detracts much from itsvalue. I find reason to differ from them on a few points, bu tit is quite possible tha t this may be due to our having workedon different genera.

The only other paper on this subject which I am acquaintedwith is by G. H. Parker,3 who treats at some length of thedevelopment of the centra l and lateral eyes. I had worked atthis point before the appearance of his paper, and on the wholeagree with his conclusions. The.se are briefly that the latera leyes are monostichous, being formed from the hypodermiswithout invagina tion. The median eyes, on the other hand,are formed by invagination, and are therefore three-layered,all the layers being derived from the hypodermis. The retinais the second layer, t he third being reduced to a post-retina lmembrane. The ma terial at Mr . Pa rke r's disposal did notenable him to go back to the commencement of the formationof the central eyes and their connection with the cerebralinvaginations.

1 Metsobnikoff, " Embryologie des Skorpions," ' Zeit. f. wiss. Zool.,' 1870.a Kowalevsky and Schulgin, " Entwickelungsgeschichte des Skorpions,"' Biol. Centralblatt,' Bd. vi, 18S6.3 Parker, "The Eyes in Scorpions," 'Bull. Mus. Comp. Zool., Harvard,'vol. xiii.


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108 MALCOLM LAUEIB.

THE OVARY AND OVARIAN EGG.T he ovary consists , as is well know n, of thr ee lon gitud inaltubes connected by transverse anastomoses, so as to form eightqua drila teral mesh es. Th e oviducts arise from the late ralangles of the two foremost meshes and ru n forward t o opentogeth er on the genital operculum. The ovary appears to beembedded in the liver, the chief mass of which lies dorsal toi t ; this is not really the case, for, tho ug h lobes of the liverpass thr ou gh the meshes of the ovarian netw ork they do n otunite on its ven tral side. Bo th the long itudin al and transv ersetube s bear ova, which project from thei r o uter surface asoval bodies each attac hed by a short pedicle and measu ringwhen ripe about V% mm . in le ngth and '83 mm . in brea dth.Ova in all stages of developmen t are present on th e o variantub es at th e same time , and the re are in addition the corporalutea (v. p. 111).Th e microscopic str uc tu re of the ovarian tubes i3 shown inPI. X I I I , fig. 2. They are the re seen to be formed of twolayers surrounding an irregular lumen. The outer layer, o. I.,

which is purely skeletal in function, consists of irregularlypolygo nal cells, with circular nuclei and strongly mark ed celloutlin es. Th e con tents of thes e cells are highly refractive.Tow ards the insid e of th is layer th e cells becom e flattenedso as to form a distinct, cellular, limiting1 layer. The innerlay er, w hich surro und s th e lume n of the tub e, is formed ofvery long and thin columnar cells, with oval nuclei and clear,faintly gr anu lar proto plasm . Th e nuclei are for the most partconfined to a cen tral zone, leaving a large part of th e oute rand a sma ller pa rt of th e in ne r en ds of the cells clear. I t isfrom this inn er la yer of cells th at th e ova and th ei r folliclesare formed; it is, in fact, the germ inal epithe lium .The first sign of the formation of an ovum is that one ofthe cells of the inner layer of the ovarian tube begins toinc rea se in size (fig. 1, ov.). It contains finely granularprotoplasm, a large and distinct oval nucleus, and a darkly


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THE EMBRYOLOGY OP A SCORPION. 109stainin g nu cleolus . Ther e is at first no sign of its presenceon the outside of the ovarian t ub e. As it increases in size,however, it pushes its way, at the head of a column of cells,towards the outside. The outer layer of the ovarian tub ebecomes very thin, but remains as a membrane containingfew, if any, nuclei (fig. 2, fol'.). By the time the ovum isabout -04 mm. in length (fig. 2) it has passed completelythrough the outer layer and is visible as a small protuberanceon the surface of the ovary. I t rem ains connected to the innerlayer of the tube by a column of cells which is somewhatexpanded over the base of the ovum. The nucleus has notincreased in size in proportion to the growth of the cell.

The nuclei of the cells of the column which connects th eovum to the inner layer of the ovarian tube next arrangethemselves so as to leave a clear space down the centre of thecolumn (PL X I II , fig. 3, mi.). They also grow round the ovumso as to form a follicle (fig. 3, fol.) one cell thick . Thecells of this follicle rapidly become flattened and their nucleibecome sma ller. The cells which remain c lustered at the baseof the ovum (fig. 3, ffer'.) on the other hand increase iu size,and shortly after the stag e repre sen ted in fig. 3, which is adrawing of an ovum of about *1 mm . in diameter, they beginto secrete the yolk of which the greater part of the ripe egg iscomposed. The outer layer of the ovarian tub e can stillbe traced as a thin and appare ntly structureless mem brane(fig. 3, fol'.) surro und ing the egg o utside the cellular follicle.The nucle us has increased in size and is now a dist inct ovalbody with a round, granular nucleolus.

In fig. 4 is shown a longitudinal section of an egg of about4 mm. in length and -28 mm . in b readth . A considerablequ ant ity of yolk is now present in the form of spheresranging iu size from mere granules up to as much as "025 mm.These spheres are clear, homogeneous, sharply defined bodiesshowing no internal structure except that there is, in thelarger ones, a collection of granules at one point nearthe outside . Roun d the nucleus the yolk-spheres are small,and round the margin of the egg the protoplasm is coarsely


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1 1 0 MALCOLM LAU RIE.granular, the rest of the space being filled up with the largerspheres.

The nucleus (fig. 4, n.), which retains its central position,is large ('05 mm.) b ut ind istin ct in o utline and is probablybreaking down as I have been unable to find any trace ofit in eggs larger tha n tha t here figured. The nucleolus(fig. 4, n'.)} which is situated towards one side of the nucleu s, isalso large, staining darkly with c armine and showing a verydistinct circular o utline. I t often contains one large , clear,circular vesicle and a number of smaller ones.The whole egg is surrounded by a distinct, rather thickvitelline membrane (fig. 4, v. m.). N o trace of pores or anyother struc ture was made out. Outside the vitelline mem braneth e egg is surrou nded , except at the base, by the follicle inwhich the two layers (fol. an d fol'.) of the ovarian tube canstill be trac ed. Th e cells of the inner laye r of the follicle arenow flattened and small. The large yolk-forming cells at thebase of the egg {ger'.) have increased in size and arrangedthemselves in a circle the centre of which is occupied by aprolongation of the ovarian tube (mi.'). The egg is only

separated from this prolongation of the lumen by the vitellinemem brane. The spermatozoa are thu s enabled to reach andfertilize the egg while it is still in its follicle.PI. X I I I , fig. 5, shows the base of a ripe egg attached to theovarian tu be . The pedicle has become shortened and its lum enhas increased very mu ch in size. The yolk-forming cells havedegenerated, their flattened nuclei (ger'.) being, however, stilldistinguishable, and the follicle has become much th inn er owingto the growth of th e egg. The egg itself is a mass of tigh tlycompressed yolk-spheres, among which I have in vain soughtfor th e nucleu s. I t is probable, however, th at the nucleus andthe greater part of the protoplasm migrate to the base of theegg as segmentation commences there.The yolk (PI. X I I I , fig. 6) consists of spheres, rang ing up to2 mm . in diameter. They are not homogeneous, bu t containspherical or prism atic bodies , which stain darkly with boraxcarmine . These bodies are very large in the smaller yolk-


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THE EMBRYOLOGY OF A SCORPION. I l lspheres, which contain one, two, or more of them. In thelarger spheres they are much m ore numerous and muchsmaller. Many of the spheres show round holes as if th edarkly staining bodies had dropped out. I t may be, however,th at these cavities contained a fatty or oily substance , whichhas been dissolved out in the course of embedding andmounting.

The only structures remaining to be described in connectionwith the ovary are the corpora lutea men tioned above (fig. 7).These are irreg ular ly shaped bodies of about '12 m m. indiam eter, showing a slight tendency to radiate struc ture , andcontain ing a considerable num ber of nucle i, which are scatteredabout withou t any definite arra ngemen t. They project fromthe surface of the ovarian tubes, and are evidently the collapsedrem ains of the follicles after th e egg has passed out. I wasconfirmed in my idea th at these were corpora lu tea by the irresemblance to the stru ctur es described by v. Siebold1 in theovary of Apus. They differ from these la tt er , however, i n notcontaining fluid.

F IR S T PERIOD.Formation of Blastoderm.The egg is fertilized in the follicle, from which it does not

begin to pass out un til the end of this period. I t then passesinto the ovarian tu be in which it undergoes the rest of itsdevelopment, the young when born being exactly like theparent in form. Kowalevsky and Schulgin2 state that the eggin Androctonus is not fertilized un ti l it has entire ly left thefollicle, and passed in to th e ovarian tub e, or, as he calls it,uterus. I can hard ly believe this to be the case, bu t it is qu itepossible th at it leaves the follicle at an earlier s tage inAndroctonus than in Euscorpius.

Stage A.I have not , unfortu nately , been able to observethe processes of fertilization and the formation of the firstsegmentation-spheres. I should think it probable th at th e

1 v. Siebold, 'Beitrage zur Parthenogenesis der Arthropoden,' Leipzig,1871, p. 191.2 Loc. cit., p. 526.


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11 2 MALCOLM LAURIE.greater part of the protoplasm with the nucleus collects at thebase of the egg. The youngest stage in my possession is shownin surface view in P I . X IV , fig. 8, and insectio hinf ig.9. The blas-toderm forms a circular patch abo ut "2 mm . in diam eter, lying onthe surface of the yolk at the end of the egg nearest to the micro-pyle, and consists of about twenty large cells, those in the centremeasu ring ab out *03 mm . in diam eter. In section (PI. X IV ,fig. 9) it is seen to be a single laye r, th e cells of which areabout -023 mm . thick in the centre. Round the marg in thecells are wedge-shaped so th at th e bla stoderm lies flush withthe surface of the yolk. The cell-contents are coarsely granu-lar, rath er more so towards th e lower side. The nucle i arelarge, round and granular with distinct outlines.

The yolk-spheres under the blastoderm appear to be breakingdown. The blastode rm and yolk are closely surrounded bythe structureless vitelline mem brane (v. m.). This stage seemsto be a little young er th an th at figured in Metschnikoff's paperin PL XIV, fig. 6.Stage B.In the next stage (PI. XIV, fig. 10) the blastodermis somewhat larger, measu ring '23 mm . in diam eter. Theblastod erm is now almo.st twice as thick ("045 mm .). Some ofthe cells are columnar, and occupy the whole depth of theblastoderm, but the majority have divided in a plane parallel tothe surface, so that it is in places two or even three cells deep.The nuclei vary in shap e, those in the colum nar cells beingoval.Stage C.In the next stage (PI. X IV , fig. 11) the b lastoderm,now -3 mm . in diame ter, is formed of an irregu lar mass of cellsshowing as yet no trace of arran gem ent into layers. The cells

are comparatively small with well-marked outlines and largenuclei. Round the margin of the blastoderm the cells form asingle layer on th e surface of the yolk, b ut in the centre theblastoderm is five or six cells thick , and the cells push the irway in between the yolk-spheres to which some of th e cellsattach themselves. These cells, which attach themselves toyolk-spheres, lose the ir definite outline and tak e, as far as Ihave been able to ascertain, no part in the further growth of


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THE EMBRYOLOGY OF A SCORPION. 113the embryo. There is no doubt that these yolk-cells arederived from the blastoderm in this and the next stages, anddo not arise in the yolk by any process of free cell-formation.Kowalevsky is also of this opinion. The yolk in the Scorpion'segg shows no sign of segmenting as does that of the Spider. Theyolk of the Spider's egg seems1 to represent the hypoblast, andtakes an active part in the building up of the embryo; that ofthe Scorpion, on the other hand, remains throughout develop-ment an inert mass of food-material. This fundamentaldifference in the segmentation makes any comparison of theearly stages of these two groups impossible, and wouldseem to point to an independent origin for their abundance offood-material. If the segmentation in Scorpions is a modifi-cation of the centrolecithal type, as would seem probable fromthe modes of segmentation in other groups of the Arachnida,it is a very extreme one, and almost all trace of its origin hasbeen lost.SECOND PERIOD.Formation of the Three Layers and the Em-

bryonic Membranes.Stage D.It is difficult to get good sections at this stage asthe blastoderm is often humped up at the end of the egg and

compressed by the ovarian tube into which it is beginning topass. In one, and only one, series of sections I have seen whatappeared to be a longitudinal groove in the blastoderm. Thisprimitive groove is figured by Metschnikoff (PJ. XVII, figs. 2and 3), but he may have been misled by the edges of the serousmembrane which is growing up and might easily give theappearance of a groove in surface view. If the primitive grooveexists, which I am inclined to doubt, as the appearance in mysections may have been due to shrinking, it is a very temporarystructure. Towards the posterior end of the blastoderm thecells are proliferating and forming what I shall call the primi-tive thickening1. From this primitive thickening is formed themass of hypoblast which is found later on in the tail-segment.

1 Locy, " Observations on the Development of Agelina nsevia," 'Bull.Mus.j Harvard,' vol. xii.


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114 MALCOLM LAURIE.It would seem to represent a modified imagination, and is com-parable to the primitive streak in the chick. I was at first in-clined to call this the primitive cumulus, but considering thefundamental differences between Scorpions and Spiders, and alsothe fact that, while Balfour1 places what he calls the primitivecumulus at the posterior end of the embyro, Locy2 gives thesame name to a thickening at the anterior end, it seemed betterto avoid a term which might suggest erroneous homologies.

A layer of cells (fig. 12, pr. hy.) is seen to be forming underthe rest of the blastoderm, though not yet extending to itsedges. This is well marked in the next stage, and forms thegreater part of the primitive hypoblast or hypomesoblast. I twould seem to be simply split off from the epiblast. I haveseen no appearance of a " down-sinking " of cells to form thehypoblast, such as is described by Kowalevsky and Schulgin ; sbu t, without the help of figures, it is not easy to be certainof their exact meaning. Wh eth er this " down-sinking" issupposed to take place over the whole blastoderm or onlyat the primitive thickening is not clear from the ir descrip-tion.

Round t he edges of the blastoderm a single layer of largecells (fig. 12, s. m'.) is seen to be spreading a lit tle way over thesurface of the yolk. These peripheral cells, which are at presentcontinuous with the epiblast, form later on the continuation ofthe serous membrane. This serous membrane, or outer layerof the amnion, is seen growing up as a single layer of cellsfrom the edges of the blastoderm (PI. XIV, fig. 12, s. m.).I t spreads over the surface of the blastoderm from all sides,and its edges ultimately meet and fuse in the middle line. Atthi s stage the edges have not yet come together, and the cellsof the layer are still small and similar in appearance to thoseof the rest of the blastoderm.

The yolk is broken down to a considerable extent, and the1 Balfour, " Notes on the Development of the Araneina," ' Quart. Journ.

Mior. Sci.,' vol. xx, 1880.3 Locy, loc. cit.3 Loc. cit., p. 526.


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THE EMBRYOLOGY OF A SOOEPION. 115cells in it (fig. 12, y. c.) are num erou s. The ir nucle i are verylarge and gran ular, and of irregu lar shapes. The cell-outlineshave entirely vanished, the cells being swollen up by an enor-mous quantity of yolk-stuff. According to Kowalevsky andSch ulg in these cells are capable of amoeboid movem ents. Cellscontinue to be added from the under surface of the blastodermto those already in the yolk up to th e end of this stage. Th eirfunctionof breaking down the yolkis carried on at a laterperiod by the hypoblast.

Stage E.In the next stage the blastoderm (PI. XIV,fig. 13) has assumed an oval form, the thick ened pa rt orventral plate measuring "35 mm. in length and "25 mm. inbreadth, though the peripheral cells extend some way beyondthis. I have not been able, either in surface view or section, tofind any trace of the primitive groove, and imagine that, if everpresent, it has filled up. The primitive thickening (fig. 14, pr. t.)is better developed tha n in the last stage, and the single layerof primitive hypo blast (figs. 14 and 15 , pr. hy.) is now quitedefinite and extends a little way beyond the thick pa rt of theblastoderm, and forms a layer {hy'.) of cells under the peri-pheral cells. These last (s. m'.) extend a good deal further thanin th e last stag e. The serous membrane (s. m.) is now com-pleted over the surface of the ventral plate.Stage F.In the next stage the embryo, of which fig. 16shows a longitudinal section, consists of two somitesthosewhich will afterwards bear the chelicerse and chelaein additionto the head- and tail-seg men ts. Th e head- and tail-segmen tsare la rge, and a third somite is begin ning to be formed fromthe tail. The first somite is smaller th an the second, and no tas yet very distinctly marked off from the head. It does n otbecome fully separated from th e head un til a much later stage(eight somites). Ex cept for this curious delay in the forma-tion of th e first, all the somites a re formed and separ ated inregular succession from the tail-segment.The epiblast has undergone little change since the last stage,except that it is somewhat thinner between the somites than inthe m . I t is begin ning to grow up at the edges over the surface


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11 6 MALCOLM LAU RIE.of the ventral plate as a single layer of flat cells to form theinner embryonic membranethe amnion proper (fig. 16, am.).This amnion never loses its connection with the epiblast as-the serous mem brane has now do ne, but rema ins attache dto its edges and only extends rou nd the egg as the epiblastextends.

The most important change in this stage is the formation ofthe mesoblast (mes.). This layer is formed under the wholeventral plate by a multiplication of the cells of the primitivehypoblast, from which it is in places not yet distinguishable.The mesoblast extends across the whole ventral plate fromside to side, and is muc h thick er in the somites tha n betweenthem.

The serous membrane (s. m.) ha s, as men tioned above, nowlost all connection with the blastoderm, and is continued roundabout two thi rds of the egg by th e " peripheral cells," whichare now beginning to separate from the egg and form a definiteme m brane . The cells of the serous mem brane are becominglarge and flat.The hypoblast extends a little way beyond the ventral plate,forming a single layer of cells (%.) in the periphery of theyolk immediately under the serous membrane.By the time the em bryo has reached a stage with three somitescompletely formed (PI. X IV , fig. 17) most of the changes whichwere going on in the last stage are completed. The amnion hasentirely closed over the embryo (fig. 18, am.), though its cellshave not yet attaine d their characte ristic form. The mesoblast(mes.) is en tirely separated from the hypoblast, and rem ainshenceforth a distinct and indepen dent layer. The hypoblast

(hy.) is now a single layer, extending under the whole ventralplate, except in the tail-segment, where it consists of a spheri-cal mass. This hypoblastic mass in the tail-segmen t is thedirect product of the primitive thickenin g. The hypoblastextends somewhat further round the egg than the otherlayers, as is diagrammatically shown in fig. 19.The description given above of the mode of formation of theserous mem bran e and am nion differs very consid erably from


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THE EMBRYOLOGY OF A SCORPION. 117th at of Kowalevsky and Sc hulg in. They describe it as a fold, theoute r layer of which forms th e serous memb rane while theinne r forms the amn ion. This is probably th e mo re prim itivemode of origin for these stru ctu res, and the mode describedabove for E. i t a l i c u s is probab ly derived from it either by ahastening of the formation of the serous membrane or a retarda-tion of tha t of the amnion . I am unab le to confirm their state-ment that mesoderm cells are present between the two layers.

THIRD PERIOD.Up to the Formation of Nine Somites.This period covers the rest of the time before the appendagesbegin to form. Th e egg has by this time entirely passed intothe ovarian tub e. I t has also increased considerably in size,bu t I am unab le to say wheth er this is due in any degree toabsorption of fluid or whether it is entirely due to internalchanges.Stage G.In the first stage belonging to this period whichI have examined (PI. XV, fig. 20) the embryo consists ofnine somites. The first of the se tha t which will bear thechelicerse, is much smaller than the others, and is seen in section

to be no t yet fully separated from the head . Th e secondsom ite, which will bear t he chelae, is larg er th an thos e followingit. The ne xt four are the am bulato ry, and the seventh willbear th e gen ital operculum . A slight groove (n. g.) runsdown the midd le line of th e bo d y; this is chiefly due tothe mesoblast having divided into two longitudinal bands(figs. 21 and 22, me*.).The epiblast is mod erately thick in t he somites, and isbeginning to grow as a single layer round the rest of the egg

(fig. 21, ep'.), carry ing the amnion with it. By this stage ithas extended almost as far as has the hypoblast. The cells in th emiddle line show a more definite arrangement than the rest ofth e epiblast. This is pre par ato ry to the formation of thene ura l groov e. Th e cells of the amnion (am.) have developedtheir characteristic nucleispindle-shaped in sestien'andform a well-marked thin membrane lying close over the embryo.The mesoblast (figs. 21 and 22, mes.) shows most iinpor-


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118 MALCOLM LAURIE.tant changes. As mentioned above, it has now separated intotwo longitudinal bands. This separation does not extend intothe tail-segment (fig. 23 , me*.), where the mesoblast remainsas a solid mass of cells somewhat thin ner in the middle line.The ccelomic spaces are now formed by a splitting of the meso-blast in the somites. They are best seen in the posteriorsomites (fig. 21, cm.), where the mesoblast is thin and formsonly a single layer on each side of the coelomic space. Fu rthe rforward (fig. 22) the mesoblast is thicker and the coelomicspace is not so well marked.

The hypoblast has undergone very little change. I t is stil lvisible in the tail as a solid mass (fig. 23, hy. m.), and spreadsunder the ventral plate and a little way beyond its margin asa single layer (figs. 2123, hy.). The cells of this single layerhave large oval nuclei which stain less darkly than those ofthe epi- and meso-blast. These nuclei are somewhat widelyseparated from each other, and the cells seem to contain aconsiderable amount of food-stuff.

The serous membrane (figs. 2123, s. m.) is by this time quiteseparate from the egg all round. I t has atta ined its finalstructure, the nuclei being enormously large ( -05 mm.), flat,and at a considerable distance from each other. As far as myobservations go I can confirm Blochmann's statement 1 that thenuclei of the serous membrane divide directly without formingany karyokinetic figures. As the serous membrane plays apurely passive part in the future development i t will not benecessary to refer to it again.

Stage H.In the next stage (PI. XV, fig. 24), which isthe last before the formation of the appendages, the embryoconsists of nine somites. The first is very much smaller thanthe o thers, while on the second, which is the largest, a traceof the appendages is ju st visible. The first six somites areclearly distinguished from those further back, owing to theirsloping backwards and outwards, while the posterior ones areat right angles to the axis of the embryo.1 " Ueber direkte Kerntheilung in der Embryonalhiille der Skorpione,"' Morph. Jahrb.,' vol. x.


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THE EMBRYOLOGY OP A SCOBPION. 119A dis tinc t groo ve, the neu ral groove (re. g.), runs down themiddle line and extends some distance into the head-segment.It is due to a thinning of the epiblast in the middle line(figs. 25 and 26, re. g.~). The ventral nervous system isformed by a thickening of the epiblast along each side ofthis groove.The epiblast now spreads as a single layer beyond thehypoblast {ep'.) and extends over nearly half the yolk, carryingth e amnion w ith it. Th is is diagra mm atically shown in fig. 27 .In the head-segmen t (fig. 25) th e epiblast is irregu larly

grooved and thick ened . This is the comm encem ent of theformation of the cerebral ganglion. In th e thoracic som ites(fig. 26) th e epib last is very thi ck and solid at the co rne rs (ap.)where the appendages are about to appear. I t is also some-what solid ju st at each side of th e neu ral groove (n. th.).This is the commencement of the thickening which will formthe ventral nervous system.The mesoblast is a thin layer in the head-segment (fig. 24,mes.), but shows the ccelomic space (cce.) distinctly. Thisdevelopment of a head coelom does not, of course, as Balfourhas pointed out, necessarily indicate that the head-segment isequ ivalent to a body somite. In t he body somites (fig.26) th e mes oblas t is pr ett y thick and t he coelomic space isalmost entirely closed up . The meso blast does not extendacross the middle line or beyond the limits of the ventralplate .The hypoblast (figs. 25, 26, hy.) shows no change from thelast stage but remains as a single layer, except in the tail-segment, where the hypoblastic mass is distinctly visible.As the next stage shows the commencement of a largenumber of new structures, the ventral nervous system, theappendag es, & c, it seems advisable to give a short sum maryof what has taken place so far.

First Period.(1) The blastoderm commences as a single saucer-shapedlayer of cells at one end of the egg (Stage A).


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120 MALCOLM LAURIE.(2) These multiply and form a thick mass (Stages B, C).

Second Period.(3) The serous membrane grows up from the edges of theblastod erm over its surface as a sing le layer of cells, and iscontinued round the yolk by the peripheral cells (Stages D F) .(4) The hypo-mesoblast is formed partly as a single layer ofcells split off from the under surface of the blastoderm andpartly , at the tail end, as-a thick mass, the primitive thick enin g,which probably represen ts an invagina tion. Before and upto this stage cells pass from the blastoderm into the yolk(Stage D).

(5) The mesoblast is form ed as a laye r several cells thick ,extending right across the blastoderm . The hypoblast rem ains,after the formation of the mesoblast, as a single layer, exceptin the region of the primitive thickening, where it is a sphericalmass (Stage E).(6) The amnion is formed as a single layer of cells growingup from the edges of the epiblast, w ith which it retain s itsconn ection. The serous membrane has by this time lost allconnection with the blastoderm, and spreads round the greaterpart of the yolk (Stage P).The embryo by this tim e consists of three somites and thelarge head- and tail-segm ents. The somites are formed fromthe tail in regular succession.

Third Period.(7) The mesoblast divides int o two lon gitu dina l ban ds, andccelomic spaces are formed in the somites and in the head(Stage G).(8) T he epiblast and amnion begin to spread round th e eggbeyond the limits of the ventral plate (Stage G).(9) Th e neural groove is formed by a thinn ing of the epiblastin the middle line (Stage H).(10) Th e epiblast in the head-segm ent begins to thick en toform the cerebral nervous system (Stage H).


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THE EMBRYOLOGY OF A SCORPION. 121

FOURTH PERIOD.From the Formation of the Appendages to theHatching of the Embryo.Stage I. Th e first stage of this third period showsasmentioned abovethe commencement of some of the most im-po rtan t stru cture s. The emb ryo, of which a surface view isgiven in PI . X V , fig. 28, now consists of twelve som ites inaddition to th e head - and tail-segm ents. These somites a reno longer separate thick enin gs as. in the last stage , bu t havegrown close up to one ano ther, and are mark ed off by narrowgrooves. The epiblast extends as a single layer all round theegg. Th e long itudina l ne ura l groove is well mark ed andextends the whole length of the body with the exception of thetail-segment.The first six somites bear appendages, i . e . th e chelicerse,chelse, and four pairs of walk ing legs. These appendages aresimple outgrowths, and are, with the exception of the first twopai rs, of app roxim ately equ al size. Th e chelicerse are muchsmaller, and the chelse somewhat larger than the other appen-

dages. The appendages are an outp ush ing of the epiblast andthe outer layer of mesoblast or somatopleure (PI. XVI, fig. 31).They are hollow, the spaces being prolongations of the coelomicpouches. There is at this stage no sign of appendages on thesomites behind those bearing the walking legs.The embryo has a stron g dorsal flexure so t ha t the cephalicsegment curves rou nd the end of th e egg. This is best seen inlongitudin al section (PI. X V I, fig. 29 ). The anterio r ma rginof the cephalic segment is deeply cleft in the middle line, thesegment being thu s divided into two lobes. The lobes are inmuch the same state as in the last stage, and show no signs ofthe cerebral invagination from which a greater part of the brainis formed. In the middle line, and a very s hort way be hindthe botto m of the cleft, is a circular raised area with a pit inits centre (PI. XV , fig. 28, st.). This pit is th e stomodaeum.It is seen in section in PI. XVI, fig. 29, and is a simple inpush-ing of the epiblast.VOL. XXXI, PART II . NEW SER. 1


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122 MALCOLM LAU RIE.Th e ventral nervous system co nsists of a pair of thicke nedbands of epiblast running the whole length of the body on eachside of the ne ura l groove (PI. XV , fig. 28). Th e bands arecut up into blocks by the grooves which separate the somites.The epiblast is no t evenly thickene d, but the nuclei are ar-ranged so as to prese nt a wavy outlin e. This is characteris ticof the formation of nerve-tissue in this animal, and was wellseen in the cerebral lobes in the last stage (PI. XV, fig. 25).Th e sma ll ganglia of the cheliceral somite are well seen atthis stage (fig. 28, g, I ) .The tail-segment, from which the six caudal somites have yetto be formed, has begun to be pushed out (PL XVI, fig. 29).The epiblast in this region is very thick, and the cavity of theoutp ushin g is lined by a thick layer of hypoblast, which is the" hypoblastic mass " of earlier stages (fig. 29, hy . m.).Besides this mass in the tail-segment the hypoblast extendsas a single layer round the whole egg (PI. XVI, fig. 29, hy.).Along the ventral side the cells of this layer are close together,hut towards the sides and back they become more scatte red,and are to a great extent involved in the yolk. It is from themass in the tail-se gm ent th at th e mesenteron is chiefly formed.The hypoblast along the ventral surface also takes some part inits formation, but that round the sides and back is not involved,though it aids in the formation of the great digestive gland orliver.The mesoblastic bands (PI. XVI, fig. 31) are not yet unitedacross th e midd le line . Th e ccelomic spaces (PI. X V I, figs. 30and 31) are well marked and quite separate for each segm ent.Those in th e first six somites are prolonged into the appendages.

The somatopleure is several cells thick ; th e splanchnopleure, onthe c ontr ary, consists of a single layer of cells. The mesoblastin the cephalic segment is thin ne r tha n in th e body somites,and the coelomic space is narrower.Stage K (PI. XVI, fig. 32).The thoracic appendages haveincrease d very much in size, and the chelicera? and chelae areboth bifurcated at th e extre mity . A section throu gh the baseof one of the ambulatory appendages (PI. XVI, fig. 33) shows


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THE EMBRYOLOGY OF A SCORPION. 123a well-developed process extending inwards towards the middleline. This is undoubtedly the sternocoxal process, which ispresent on the second, third, and fourth appendages of theadult. Lankester1 characterises the presence of this processas a very important point of resemblance between the thoracicappendages of Lim ulus and Scorpio. I t is therefore inte res t-ing to find it at this early stage present on all four pairs ofam bulatory appendages. A series of sections thro ug h the baseof the fifth appendage, i. e. third ambulatory (PI. XVI, fig. 34,ah), shows the first stage of another structure characteristicof Lim ulus and the Ara chnid s the coxal gland. This consistsof a simple tube opening to the exterior at the base of the fifthappendage (fig. 34 a) , and running forwards through the meso-blast to open in fig. 34 h into th e ccelomic space. The re can beno doubt th at it is a nep hridium . Grulland's researches3 onthe coxal gland in the young Limu lus point to the same con-clusion. I have been unable to find traces of nephridia inany other somites, unless, indeed, th e genital tubes are partlynephridial. The six abdominal segments also bear appendages(PI. X V I, figs. 32 and 35). These appear on surface view muchmore prominent than they really are owing to their whitecolour, which is due to the greater th ickness of cells. Insection (PI. XVI, fig. 35) they are seen to project very slightly,and to be formed by a thick enin g of th e epiblast and som ato-pleur e, bu t with no definite o utpush ing such as ther e is in th ethoracic appendages. The first pair of these appendage s thegenital operculais very small, and concealed by the last pairof walking legs. The other five pairs the pectines and fourpairs of lung-booksare all of approximately equal size andstru ctu re. I have been unab le to find the smallest trace ofappendages on the somites behind these, i.e. somites 13 17,and do not believe they exist.

The cephalic segment is not so deeply cleft as in t he laststage, and the mouth has shifted posteriorly so that now it liesbetween th e bases of the chelicerse. I n the cen tre of each1 ' Limulus an Arachnid,' p. 20.3 ' Quart. Journ. Mier. Soi.,' vol. xxv.


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1 24 MALCOLM LAU RIE.cephalic lobe is seen a dark spot (fig. 32, ce. in.). These spotsare the cereb ral invag inations . They begin in a somewhatearlier stage (PI. XVI, fig. 36) as a pair of small inpushings.These extend rapidly backwards and meet in the middle line,their two lumens becoming continuous. This is seen in PI .XVI, fig. 37 AD, in which four transverse sections throughthis region are figured. Owing to the strong cephalic flexure inthis stage the stomodaeum (st.) is also shown in section. Thecells, both at the sides of the cephalic lobes and throughout thegreater part of the invaginations, are rapidly increasing innum ber to form the cerebral ganglia. Those in the centre ofthe cerebral lobes rem ain as a thi n laye r, and take no p art inthe brain formation. The cells also on the dorsal side in th emiddle, -where the two invaginations have united (PI. XVI, fig.37 D , oc ), are m ore closely packed tha n the o thers, and t ake nopa rt in the formation of the brain. They are th e beginnin g ofthe retinal layer of the central eyes.

The ventral nervous system is in much the same conditionhistologically as it was in. the last stage. Th e comm encem entof its separation from the hypoderm is can, however, be seen(PI. X V I, fig. 35) where the hypodermis is growing over itfrom each side as a thin layer.The tail segment is now divided into six somites, and extendsforward along the ventral surface of the body, reaching, at thisstage, to the third abdominal somite. The epiblast is thickenedon th e ven tral surface to form the nervous system . This isnot shown in fig. 35, as th e section passes between two thic ken -ings. Th e cavity of the ta il is occupied by a tubu lar extensionof the hypoblast (fig. 35, hy.) surrounded by mesoblast. Ther eis as yet no trace of the proctodseum.The coelomic spaces in the thoracic somites have notdeveloped mu ch. Those in the abdominal som ites, however(PI. XVI, fig. 35, coe.), have extended enorm ously, and nowreach round almost one third of the egg. The mesoblast, ex-cept in abdominal appendages, consists of two single layers ofcells. In the tail the ccelomic spaces are not yet formed.Stage L.The embryo, of which fig. 38 (PI. XVII) shows a


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12 6 MALCOLM LAU RIE.The retina of the central eyes is still a thickening of thedorsal layer of th e cerebral invagination (PI. X V II , fig. 43 ,rtn.). It is visible in surface view (fig. 38, oc.) as a white spoton the ma rgin of the invagination. The hypoderm is imm e-diately outside it is som ewhat thic kene d, and will in thisregion form the vitreous layer (fig. 43, vit.).The ventral nervous system is now completely separated fromthe hypodermis (figs. 40 and 41, n. c ) . The cells are beginningto congregate together to form the ganglia, though the nerve-cord between the gan glia is still large ly cellula r. Nerves are

seen growing out from the ganglia as thick cords of cells(fig. 40 ). The ganglia contain a clear space in their centrewhich later is occupied by a mass of fibres.The tail (PI. XVII, fig. 38) has now attained its fullnum ber of segme nts but the sting is not yet formed. Thegut extend s up almost the whole leng th of the tail. The re isno sign yet of th e formation of th e proctodseum. The hypo-blast in the rest of th e body remain s as a scattered layer ofcells.The mesoblast has now grown ro und the body as a doub lelay er, with the ccelomic space between . In the middle lin e ofthe back, where the right and left folds of mesoblast meet,there is a somewhat irregular thickening in which both soma-topleure and splanchnopleu re seem to be involved. From thisthickened band, which extends from close behind the brain tothe beginning of the tail, the heart is formed. On the ven tralside in the thoracic region the mesoblast of the outer layer isbroken up into long strings of cellsthe musclesso that thecoelomic space can no longer be very definitely made out.Th e stomodseum reache s as far as the back of the cereb ralganglion. This is the limit of its grow th, and it remains aclosed tube u nti l, at a much later stage , the g ut has grownforward and united with it.Stage M.The embryo (PI. XVII, figs. 44 and 45) doesnot show very muc h change in surface view. The thoracicappendages are longer and distinctly segmented. They overlapacross th e middle line and conceal the pec tines . The chelicerae


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THE EMBRYOLOGY OF A SCORPION. 127are further forward in relation to the mo uth, which can nowbe seen lying between the bases of the chelse.

The genital opercula begin to grow out from the body walland the genital duct begins to be formed. This last (PI. X V II ,fig. 46) is developed in the mesoblast as a tubular portion ofthe coelom, but does not open to the exterior up to the time ofhatching. It m ay be nephridial in its natu re, but this verylate formation of th e ext ern al apertu re is n ot very favorableto such an hypothesis. The pectines are separated at theirouter ends from the body wall. Th e in push ings for the lung-books are much deeper, and the cavity, which extends forwardsfrom the open ing, is divided up by lamellae which grow downfrom its upp er end (PL X V II , fig. 47 ). I t is in close relationto a space in the mesoblast which contains blood-corpuscles.

Th e cephalic segment (PI. X V I I , fig. 45 ) is now rapidlyapproxim ating to its final shape. The cerebral ganglion, whichis seen from the surface as a four-lobed white mass (fig. 45, ce.),has now lost all connection with the epiblast. The invagin a-tion remain s, bu t its sides no longer give rise to nerve-tissue(PL X V II , figs. 48 and 49). The thickenin g for the centraleye (figs. 48 and 49 , rtn.) is more largely developed, andpig me nt is deposited in t he ends of t he cells furth est fromthe invagination . The eye is plainly visible as a doub leblack spot on the surface. The upp er edge of th e invag ina-tion is growing down to close its orifice. Th e hyp oderm islying imm ediately above it is clearly m arked off from th erest as the vitreous layer (fig. 49, vit.). A considerable spacestill separates the retina from the vitreous layer.

The lateral eyes now appear for the first time as black spots onwhat Lank ester terms the " optic area," i. e. the front marginof the head (PL X V II , fig. 45 , o c) . Their development, asPa rke r 1 has shown, is strikingly different from that of thecentral eyes. Each eye, and in this species th ere are at firstthr ee, is formed (fig. 50) by a slightly cup-shaped thick enin gof the hypo dermis. Th e nuclei of this thicken ed portio nbecome larger, and pigment soon begins to be deposited at the1 Loo. cit.


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1 2 8 MALCOLM LAU RIE.ou ter ends of the cells. Fig . 50 a shows a somewhat laterstage, in which the cupping of the hypodermis has becomeflattened out. There is no invagination of any sort, and theeyes are, as Lankester and Bourne 1 described them, mono-stichous. The ventral nervous system has not undergone muchdevelopment. I t has sunk somewhat deeper and is separatedfrom the hypodermis by the mesoblast.

The tail has now developed its terminal segmentthe sting.The cavity of this last is partly occupied by the paired poisongland, apparently formed by inpushin g of the hypoderm is (PI.XVIII , f ig . 51, p. gl.). Each mass is connected to the super-ficial hypodermis by a short duct.The gut extends down the whole length of the tail, and theproctodseum is present in the form of a solid massof hypodermiscells blocking up its end (PI. XVIII, fig. 51, prod.). The guthas also begun to grow forward (PL X V II I, figs. 52 and 53). Inthe last abdominal segment it is a complete tube surroundedby a thin layer of mesoblast (fig. 52, int.). It gives rise totwo tubu lar outgro wth s from its dorsal side, which are theMalpighian tubes (fig. 52, mlph.). These run first towards theback and the n bend forward. There can be no doubt as totheir hypoblastic origin in this form, as the proctodseum is notyet formed. They have been already shown to be outgrow thsof the mesenterou in some Spiders by Loman,2 and also in ter-restrial Amphipoda by Spencer.3 Further forward (PI. XVIII,fig. 53, int.) th e gu t is simply a semi-cylindrical layer of hyp oblastsupported by a string of mesoblast and open to the yolk on itsdorsal side. In th e tho rax it has not yet begu n to form.The mesoblast is broken up into strings and bands. Thecoelom is still pre tty d istinct in the ab dom inal region (PI. X V I I I ,fig, 53, cm.), and the hea rt is a large thin-walled tube appare ntlyconnected with bo th somatopleure and splanchn opleure. Asmen tioned above, the genital tube is formed in the somato-pleure in the seventh somite and is a portion of the ccelom.

1 ' Quart. Journ. Micr. Sci.,' vol. xxiii.3 ' Tijdschrift der nederl. Dierk. Vereen,' i.3 ' Quart. Journ. Micr. Sci./ vol. xsv.


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THE EMBRYOLOGY OF A SCORPION. 129Stage M,The changes from the last stage up to th e tim eof hatch ing are not very num erous, thoug h very im port ant.The body attain s a stru ctu re almost exactly like th at of theadult, the appendages being segmented and the whole animalcovered by a thin, structureless, highly refracting cuticle. Thecoxal gland still opens by a small apertu re to the exterior atthe base of the fifth appendage (PI. X V I I I , fig. 54). Thisape rtur e, which is lined for a short distance by th e cuticle,leads to a str aig ht duct (fig. 54) lined by cubical cells withround nuclei, which closely resemble the cells of the gland.

The gland itself is distinguishable into medullary and corticalportions as described by Professor Lankester1 in. the adult. Thetubule s have distinct lumen s surrounded by a cubical epithelium .The gland and its duct are surrounded by a thin capsule of flatmesoblast cells.The genital tubes have pushed the ir way some distancebetween the lobes of the liver, but they are not yet connectedby transverse tube s nor do they open to the exterior. The twolayers of which the tube is composed in the adult (v. supra,p. 108) are not yet distinguishab le. The pectines approximatevery closely to their adult structure.The ninth, tenth, eleventh, and twelfth appendages are alsovery similar to those in th e adult (PI. X V I II , fig. 55). Thenum ber of lamellae is not so gre at, but their st ruc tur e is verywell Bhown. Ea ch lam ella is covered by a th in cuti cle, and itscavity is in direct comm unication with a blood-sinus (fig. 55 ,bl. s.). The cells which form th e lamellae are v ery large , espe-cially towards the base of the appendage. Towards the apexthey become smaller, and finally pass into a mass of differentcells from which more lamellae are formed as the animal grows.The spaces between th e lamellse (fig. 5 5, a. c'.) ai'e narrower andin communication with the exterior through the stigma.The head is now completely formed, the m outh havingshifted so as to lie beh ind t he chelicerae. Th e in vag inatio nwhich forms the centr al eyes has closed up . A stage immedi-ately after its closure is shown in PI . X V II I , fig. 56. He re the1 ' Proc. Roy. Soc.,' vol. xxxiv, 1882-83,


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130 MALCOLM LAURIE.lips of the invagina tions have come tog ethe r, bu t n ot fused.The posterior layer of the invagination is visible as a thin layerof cells (fig. 56, rtn'.), separated from the retina by a narrowspace. The vitreo us layer (vit.) is distinctly marked as athickening of the hypodermis on the top of the head, the nucleiin that region being elongated, but there is still a small spaceseparating it from the retina . By the time the embryo ishatched the eye (PL X V I I I , fig. 57) has lost all connectionwith the hypodermis at the point where it was invaginated.The cells are long and deeply pigmented round their margins.The pigment is not equally abun dant througho ut the wholelength of th e cell, bu t five alternate ly mo re and less deeplypigm ented zones can be dis tingu ished (fig. 58). The base ofthe cells is most deeply pigm ented , and their superficial endscome next. The nuclei of the retinal cells lie in zone 4 ,but I have been unable to find any trace of either rhabdom esor phaospheres. I have also not been able to trace any migra-tion of m esoderm cells among the retinal cells. The posteriorlayer of th e invagina tion can with difficulty be made out indepigm ented sections owing to the flatness of its nuc lei, and itis absolutely undistinguishable in sections from which the pig-me nt has not been removed. This posterior layer forms thepost-retinal me mb rane of the adult eye. The optic nerve isbeginning to grow out from the cerebral ganglion, but has notyet come into connection with the eye. The hypode rmis, im-mediately in front of th e eye, is formed of a sin gle layer oflarge transparent cells with faintly staining oval nuclei(PI. XVIII, f igs. 57 and 58, vit.). This vitreous layer iscovered by a thin cuticle exactly like th at which covers th erest of th e surface of the body. The only sign of the formationof the lens is a sligh t cupping of the vitreous layer at one point(fig. 58). The hollow formed he re is, howeve r, not as yet filledup by any cuticular substance, but the cuticle passes straightover it. Rou nd th e area , where the lens will form the hypod erm isis deeply pigm ented. The cells are much smaller th an thoseof the vitreous layer, and their nuclei are irregular in shape.The cells of the lateral eyes (PI. X V II I, fig. 59) are about


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THE EMBRYOLOGY OP A SOOEPION. 131the same size as thos e of th e med ian ones. Th e pigm ent is not ,however, arranged in definite zones, thou gh it is more abu nda ntat the base and at the outer ends of the cells than in the middle.There is a small third lateral eye presen t, and the hypoderm isaround andthe lateral eyes being so to to say on the edge ofthe headbelow th e eyes is all pigm ente d. I have beenunable to find the nerve to these eyes and think it is probablyno t yet formed. Th e cuticle over the latera l eyes is no tthick ene d to form th e lens, and I h ave seen no sign of th epeculiar mode of lens-formation described by Pa rke r, 1 i . e . theends of the perineu ral cells pushing in front of the retina. I tis , of course, possible that this does not take place till later.

The bra in and ventral cord have alm ost attained theiradult stru ctur e. In the nerve-cord the re is a s tring ofcells in the middle line (PI. X V I I I , fig. 60) dorsal to thecords proper, which seems to represent the centre of the neuralgroove.Th e tail is exactly similar to th at of th e ad ult, an d is carriedin th e sam e way curved over the back. Th e poison-glands arefully formed and surrounded by muscles, but do not occupy somuch of the term inal segment as in the adu lt. The proctodseumis lined by flat cells and h as p ushe d its way almo st to th ebase of the tail. The m esentero n is fully formed only in thehind segments of the body (PI. X V II I , fig. 60 ). From theend of th e stomodaeum to wh ere th e last he pa tic caecajoin it th e inte stin e (fig. 61) is surrou nde d by a definitecylindrical layer of mesoblast which is continuous with t ha tsurrounding the lobes of the liver, but the hypoblast cellslining this cylinder (fig. 61, hy.) are not yet definitely arranged.The nuclei are scattered about in groups for the m ost par tnear the outside, and the cells are drawn out into irregu larmore or less pyramidal form, the apex of the pyram id poin tingtowards the centre of the tub e. The re is no definite lum en,th e space betwee n the cells bein g filled up by small yolk-sph eres(fig. 61,2,*.).The liver-follicles are muc h th e same in str uc tur e as the1 Loo. oit., p. 199.


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13 2 MALCOLM LAURTE.inte stin e. They contain, however, a rathe r larger proportion ofyolk . The scattered layer of hypoblast cells, which in th epreceding stages surrounded the yolk, takes a large part in theirformation. They open into the intestin e in pairs by wideducts.

The Malpighian tubes (PI. X V II I, fig. 60, mlph.) have notunderg one much development. They reach well forward in thebody, and open into the intestine in the first caudal segment.I t is evident from the s tru ctu re of the intest ine th at theyoun g scorpion does not need food for some tim e after h atc h-

ing . The large amo unt of yolk which still exists mu st last itfor some weeks, or most probably till th e next sprin g. If thisis the case embryonic life practically lasts twelve months as theeggs are fertilized in May.The outer layer of the mesoblast has now for the most partformed itself in to muscles. Th e inn er layer is very much com-plicated, being folded in so as to su rrou nd the g ut and thelobes of th e liver. The spaces between the lobes of t he liver,which are undou bted ly th e true coelom, are filled up by a ne t-work of trabecular tissue (PI. X V II I, fig. 60). The heart,

pericardium, and blood-vessels are fully formed and contain aconsiderable number of large nucleated corpuscles.Summary of the Changes during the Fourth Period.

(1) The thoracic appendages begin as simple outpushings ofthe body wall containing a portion of the coelom (Stage I).They rapidly increase in length and the chelicerse and chelaebecome bifid at the ir ext rem ities . Sternoco xal processes arepresent on the third to sixth appendages (Stage K).The chelicerse, which were at first behind the mouth, gradu-ally move forward relatively to it till they come to lie in frontof it (Stage L ).(2) T he coxal gland begins as a simple tub e opening to theoutside at the base of the fifth pair of appendages, and openingat the other end into the ccelom (Stage K ). The tub e soonbecomes coiled, bu t th e extern al opening p ersists un til afterhatching . I t is undoubtely a nephridium,


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THE EMBRYOLOGY OF A SCORPION. 133(3) The abdominal appendages appear as thickenings of theepi- and meso-blast on the seventh to twelfth somites (Stage K).Th e first pair (genital opercula) does no t develop furth er ti lla late stage (L).The second pair (pectines) form a n um ber of sh ort long i-tud ina l ridges on the surface of the abdom en (Stage K) . Th eythen separate from the body, the separation beginning at theirouter ends (Stage M).The third to sixth pairs (gill-books) begin to be pushed in(Stage L ). The inpush ing becomes deeper, and begins to be

divided up (Stage M), and by the time the embryo is hatchedthey have attained their adult condition in every respect exceptsize and number of lamellae.(4) The cerebral ganglion and central eyes begin as a pair ofinvaginations on the cephalic lobes. These invaginationsmeet in the m iddle (Stage K ). The cerebral gan glion is formedfrom the sides of the invaginations, which rapidly becomeshallower and un ite so as to open in th e m iddle line . Th edorsal surface of the invagination becomes thickened to formthe retina of the central eyes (Stage L).The brain becomes entirely separate from the hyp oderm is,the invagination rem aining to form the eyes (Stag e M ). Theinvag ination closes up and its lum en disapp ears. Th e cells ofits lower layer form the post-retinal mem brane . Those of theupp er layer form the retin a, and come in conta ct with the hy-podermis on the to p of t he h ead, which is thickened in thi sregion to form the vitreous layer. Th e retina l cells becomedeeply pigmented (Stage N).(5) The lateral eyes form as cup-shaped thick enin gs of th e

hypo dermis in th e " optic are a," th e cells of which becomepigm ented. Th ere is no in vag ination , and the y consist of asingle layer (Stage M).(6) The ventral nervous system forms as a pair of thickenedsegmen ted band s, one on each side of the neu ral groove(Stage I) . The nerve-co rds sink down, a th in layer of hypo-dermis grow ing over them . Th ere is at thi s tim e (Stage K)a dis tinct postora l pair of gan glia for th e chelicerse. Th e cells


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134 MALCOLM LAURIE.become aggregated to form gan glia, and the cheliceral gangliabecome fused with the cerebrum.

(7) The t ail grows out, lying along th e ven tral surfaceof th e abdom en. The poison-gland in its termin al segmen t isformed by a pair of invaginations of the epiblast.(8) The hypoblast consists of an irregu lar layer und er thewhole em bryo and a solid mass at the tail end (Stage I) . Asthe tail grows th e hypob last grows into it as a tub e reachingdown to the last somite (Stage K).The hypoblast forms the gut in the abdominal portion of the

body , grow ing forward in a sling of mesoblast at first as a flatlay er, which soon becomes ben t rou nd into a cylinder. Th eMalpighian tubes are formed as outgrowths from the mesenteronin the first post-abdominal somite (Stage M).The gu t does not reach forward to the stomodaeum tillshortly before hatch ing , and at this period t he po rtion ofit in to wh ich th e liver-follicles open is no t fully formed(Stage N) .(9) The stomodseum is formed early . I t lies at first in fron t

of the cbelicerse (St ag e I) , bu t soon shifts its position andcomes to lie behind them . I t e xtends inwards as far as t heback of the brain.(10) The proctodaeum is formed much later than the stomo-dEBum. I t is at first a solid plug of cells (Sta ge M ). As itincreases in size it appears to replace the hypob last in th elast four somites.(11) The mesoblast consists at first of a pair of segmentedbands with a separate ccelomic space in each somite, and alsoone in t he cephalic seg men t (Stag e I) . Th e ccelomic spaces

soon un ite, and the mesoblast bands join across th e ventralsurface. Somew hat later they extend rou nd the ccelomicspace extend ing with them and unite in th e middle lin e onth e dorsal surface (Stag e L ). Fro m the thicken ed band wherethey have unit ed on the dorsal surface th e hea rt is formed. Aportion of the ccelom in the seventh segment becomes separatedoff to form the genital tubes (Stage M ). The se do not open tothe exterio r. The outer layer of the mesoblast forms chiefly the


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THE EMBRYOLOGY OF A SCORPION. 13 5muscles of th e body. Th e inner layer becomes folded so as tosurround the liver and intestine, and the ccelomic space becomespartly filled up by trabecular tissue.

Conclusion.The development cf this Scorpion, of which I have tried togive an outline above, is inter estin g in many poin ts. I t doesnot agree closely with any other Arachnid type as yet described,and I have for the present given up all attempts at comparison.The development of the cen tral and lateral eyes entirely bearsout L ankester and Bourne 's description of their structu re. Itis true that the central eyes are three-layered, but as the retinais the second layer from the surfacethe third layer formingonly a post-retinal mem brane they may be called diplostichous.The account given above of their development agrees in allessential respects with that of Parker, but, having a largersupply of embryos, I have been able to trace the earlier stagesand the connection of the eyes with the cerebral invagination.Their mode of origin resembles very closely Locy's1 description

of the development of the eyes in A g e l e n a ns ev ia, the chiefdifference being that in Agelena the optic invaginations appearto have no connection with the formation of the brain. Locydoes not, however, give a detailed description of the formationof the latter.The description given above of the development of the lateraleyes also agrees prett y closely with th at of Pa rke r. In these,as in the central eyes, Lankester and Bourne's conclusions areconfirmed, and Patten's 3 conclusions as to w hat th e stru ctur eof the eyes must be in order to fit in with his theories areshown to be with out foundation. Th e later al eyes are mono-stichous, being simply somewhat specialised hypodermis cells.The mode of formation of the ventral nervous system isexceptional among Invertebrates, resembling rather that of

1 'Bull. Mus. Comp. Zool., Harvard,' vol. xii, p. 85.3 "Eyes of Molluscs and Arthropods," 'Mitth. Zool. Stat. Naples,'Bd. vi.


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136 MALCOLM LAUEIB.Ch ord ata. Th e nerve-coi'd inste ad of peeling off from th esuperficial layer of epiblast sinks down bodily, and is coveredby a layer of epiblast which grows over it from each side.

The d evelopmen t of the coxal gland leaves, I t hi nk , no roomto doubt that it is a nephridium . That of the genital tub es isless conclusive, bu t I should thi nk it prob able th at they arealso, at least in part, nephridial.The gill-books are undo ubtedly appendages comparable tothe abd ominal appendages of Limulus. W het her they arereally inv aginated, i . e . whether the edge of each lamella in the

Lim ulu s appendage corresponds to the bottom of th e foldbetween the lamellae in the Scorpion's gill-book, or whether thewhole appendage has become sunk in a hollow in the abdominalsurface w ithou t b eing invaginate d, it is difficult to say. U n-doubtedly, the surface now exposed to the air has always beenthe external surface, but that would be the case with either ofth e above modes of derivation . Alth ou gh the second altern a-tive has th e adva ntage th at it is easy to see how th e ch angecould take place gradually, I am inclined to think the first isprobably the t ru e way in which they have arisen. One argu-ment in its favour is that if the second alternative were correctone would expect the gill-book to commence as a distinc t out-grow th, which would become sunk in a pit. Now , the re is nosuch outg row th in the formation of the gill-book. The firstthing to appear on the thickened portion of the epiblast, fromwhich th e gill-book is formed, is a pit (PL X V II , fig. 41 ). Thelamellae do n ot begin to form t ill a later stage. Again, theabdom inal appe adages of Limulu s are directed toward s th e tailas one would expect abdom inal appendages to be. No w, if th eappendage had sunk in without invagination, one would expectit to be still directed towards the tail unless there were somevery good reason for its having changed its direction. If, onthe contrary, it had become invaginated it would naturally bedirected in the opp osite direction tow ards th e head, and thi s iswh at we find in the Scorpion. The inpu shin g is from th ebeginning towards the head, and the aperture opens towardsthe tail (PL X V II , fig. 47 ). I th ink it is quite conceivable


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THE EMBRYOLOGY OP A SOOEPION. 13 7tha t the changed conditions of development, due to terrestriallife, and the consequent pressure on the embryo, may have pro-duced this change. A detailed account of the development ofthese appendages in Limulus may throw more light on thematter, but, unfortunately, though many authors have attackedthe problem, a complete and satisfactory account of the develop-ment of Limulus is not yet in existence.

EXPLANATION OF PLATES XIII, XIV, XV, XVI,XV II, & XV III,Illustrating Mr. Malcolm Laurie's paper on "The Em-bryology of a Scorpion (E u sc o rp iu s it a li c u s ). "

Abbreviations.a. c. Air-cavity in gill-book, ac . Air-spaces between the lamellse of gill-book, ah. ap . Abdominal appendage, am . Aranion. am. c. Amniotic cavity.ap . Appendage, bl . Blastoderm, bl. s. Blood-space, bl. c. Blood-corpuscle.can. Caudal segment, ce . Cerebral ganglion, ce . in . Cerebral invagination.ceph. Cephalic segment, ex . Ccelom. cox. Coxal gland, cox. d. Duct ofcoxal gland, ep . Epiblast. ep'. Extension of epiblast beyond ventral plate.fol. Eollicle. foil. Outer non-cellular layer of follicle, g. 1. Ganglion ofcheliceral somite, ger. Germinal epithelium or inner layer of ovarian tube.ger'. Yolk-forming cells derived from germinal epithelium, ge. t. Genitaltube. hi . Heart. hy . Hypoblast. hy'. Extension of liypoblast beyondventral plate, hy. m. Mass of hypoblast in caudal segment, int. Intestine.

I. Gastric gland, mes. Mesoblast. mi. Prolongation of ovarian tube to egg.mlph. Malpliigian tubes, n. g. Neural groove, n. n'. Nucleus, nucleolus.n. c. Nerve.cord. n. gl. Nerve-ganglion. n. th. Neural thickening, oc.Central eye. oc'. Lateral eye. o. I. Outer layer of ovarian tube. ov . Ovum.p. gl. Poison-gland, pr. hy. Primitive hypoblast (hypomesoblast). proct.Proctodseum. pr. t. Primitive thickening, rtn. Ketina of central eye.rtn'. Third layer of central eye, post-retinal membrane, s. m. Serous mem-brane, s. m'. " Peripheral cells." torn. mes. Somatic mesoblast. spl. mes.Splanchnic mesoblast. si . Stomodseum. ste. p. Sternocoxal process, stg.

VOL. XXXI, PART II . NEW SEB . K.


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138 MALCOLMStigmata, ie . Telson. tr . mes. Trabecular mesoblast occupying ccelom. vil.Vitreous layer of central eye. y. c. Cells in yolk. yk . Yolk. The somitesare numbered i, n, m , &c.

PLATE XIII .FIG. 1.Transverse section of ovarian tube, showing the two layers; onecell of the inner layer enlarging to form an ovum. X *-^.FIG. 2.Transverse section of ovum and ovarian tube . The egg has nowpushed its way through the outer layer, and appears as a small protuberanceon the ovarian tube . The follicle is beginning to form from the cells of theinner layer, which have accompanied the ovum, x &^.FIG. 3.Longitudinal section of ovum of -1 mm. diameter, showing thetwo-layered follicle and the yolk-forming cells (ger',). x ^s.FIG. 4.Longitudinal section of egg of '4 mm. in length, showing yolk-spheres, indefinite nucleus , and s trongly marked nucleolus. The egg is sur-rounded by a vitelline membrane. The rest as in Fig. 3. X S-|3.FIG. 5.Section through the base of a ripe egg. x 1^s-.FIG. 6.Yolk-spheres from ripe egg, showing the darkly stained sphericaland prismatic bodies and the clear spaces. X i p .FIG. 7.Section through a corpus luteum and part of ovarian tube.

PLATE XIV.FIG. 8.Surface view of one-layered blastoderm, x -^ -.FIG. 9.Section through one-layered blastoderm, same stage as Fig. 8.x ^ .Fi&. 10.Section .through blastoderm later than Pig. 9, showing the cellsmultiplying to form a mass at one pole of the egg. x ?%&'FIG. 11.Section through more advanced blastoderm. X 4p.FIG. 12.Transverse section through blastoderm at time of formation ofprimitive hypoblast and serous membrane. The yolk and yolk-cells are drawnin detail in this figu re to show the breaking down of the former, x s-p .FIG. 13.Surface view of blastoderm now becoming oval, x -&p-.FIG. 14.Transverse section through posterior end of embryo figured inFig . 13 , showing serous membrane, primitive thickening, primitive bypoblast,and " peripheral cells." x & p .FIG. 15.Transverse section through anterior part of embryo about thesame stage, x l^- .FIG. 16.Longitudinal section through an embryo a little younger thanFig. 17, showing two somites with a third forming. The mesoblast isforming from the primitive hypoblast, the amnion is growing up from the


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THE EMBRYOLOGY OF A SCORPION. 139edges of the hypoblast, and the primitive thickening is well seen in the caudalsegment. X -^ p.FIG. 17.Surface view of embryo, with three somites fully formed.x J-.

PIG. 18.Transverse section through posterior end of Fig . 17 , showinghypoblastic mass, mesoblast, &c. X i-^.TIG. 19.Diagrammatic representation of the relative extension of thevarious layers in an embryo of the stage of Fig. 17.

PLATE XV.FIG. 20.Surface view of an embryo with seven somites, drawn as ifflattened out. a 6 and c d are the planes of the sections figured in Figs.21 and 23. X *-.FIG. 21.Transverse section through one of the posterior somites of anembryo with seven somites (a b in Fig. 20), showing the three layers,epiblast thinning in centre, and mesoblast t hi n; amnion, serous membrane,and ccelomic spaces. X *\9-.FIG. 22.Transverse section through one of the anterior somites of Fig.20. x -4*.FIG. 23.Transverse section through tail-segment (cd) of Fig. 20,showing the undivided mesoblast and the hypoblastic mass, x 4 ^ .FIG. 24.Surface view of embryo of nine somites, drawn as if extended.X 3SL.FIG. 25.Transverse section through head-segment of Fig. 24, showingepiblast thickening to form cerebral nervous system and spreading (ep'.), withthe amnion beyond the ventral plate, neural groove, thin mesoblast, with smallccelomic space and hypoblast. X i-\Q-.FIG. 26.Transverse section through one of the anterior somites of Fig.24, showing the epiblast very solid where the appendage will develop (ap.)aud form the neural thickening {n . th.) at each side of the neural groove.Mesoblast thick, and ccelom not very eviden t, x x\s-.FIG. 27.Diagrammatic representation of the relative extension of thevarious layers in an embryo of the stage of Fig. 24.FIG. 28.Surface view of embryo at Stage I (ten somites) extended in aplane, showing appendages, cheliceral ganglion, stomodseum, &c. X A&..

PLATE XVI.FIG. 29.Longitudinal section of Stage 1 in the middle line, showingdorsal flexure of the embryo, commencement of tail outgrowth, stomodseum,&c. x -^ . .


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140 MALCOLM LAURIE.PIG. 30.Longitudinal section to one side of the middle line, showing the

appendages and coelomic spaces. X -.FIG. 31.Transverse section through the third somite of Stage I, showing

the formation of the appendage, the neural thickening, &c. X -e--.FIG. 32.Surface view of embryo at Stage K, showing the cerebral imagi-

nations, abdominal appendages, tail, &c. X -^-.PIG. 33.Transverse section through the base of a thoracic appendage,

showing the sternocoxal process.PIG. 34, aA.Series of sections thiough base of fifth appendage, showing

the coxal gland.PIG. 35.Transverse section through one of the abdominal appendages and

the tail, showing the appendage, the neural thickening beginning to separatefrom the epiblast, the gut forming in the tail, &c. X -715-FIG. 36.Transverse section through the cephalic segment of a somewhatearlier embryo, showing the beginning of the cerebral invagination. x - ^ .

PIG. 37, AD.Sections through the head of an embryo of Stage K, show-ing the cerebral-optic invaginations. X -^s..

PLATE XVII.PI G 38.Surface view of an embryo of Stage L extended in a plane,

showing the cerebral invagination, the central eyes, &c. X ~-.PIG. 39.Transverse section through the base of the fifth appendage,showing the coxal gland, x -2^-PIG. 40.Section through the pectines. X ^ .PIG. 41.Section through an abdominal appendage, showing the inpushing

to form the gill-book. X -',5-.PIG. 42, a, b.Transverse sections through the cerebro-optic invaginations.x -^a-.FIG. 43.Longitudinal section through the cerebro-optic invaginations,

showing the formation of the brain and the central eye. X A-j- .PIG. 44.Surface view of an embryo of Stage M, from the ventral surface.

X 4j2..PIG. 45.Surface view of the dorsal side of the head of the same embryo,

showing i he central and lateral eyes, x \0-.PIG. 46.Section through the seventh somite, showing the formation of

the genital tube from part of the ccelom. X sf-.PIG. 47.Longitudinal secion through a gill-book, showing the commence-

ment of the formation of the lamellee. x :L\^.FIG. 48.Longitudinal section through the head end, showing the stomo-

deeum and the cerebro-optic invagination from which the brain is now entirelyseparate. X -*T*-.PIG. 49.Transverse section through the same region, x ^-.


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THE EMBRYOLOGY OF A SCORPION. 14 1FIG. 50.Longitudinal section through the lateral eye, showing its forma-tion by a thickening of the hypodermis. X i - p .PIG. 5O.Section through a somewhat older lateral eye in which theinpushing of the hypodermis has disappeared, x ^s.

PLATE XVIII .FIG. 51.Longitudinal section through the tail end of Stage M, showingthe poison-gland, proctodseum, intestine, &c. X 4A.FIG. 52.Transverse section through the posterior end of the body, showingthe intestine, with the Malpighian tubes, the heart, &c. X -^ -.FIG. 53.Transverse section a little further foward than Fig. 52, showingthe intestine, which has not yet closed into a tube. X - ^ .FIG. 54.Section through the coxal gland of a newly-batched scorpion,showing the opening to the exterior, &c. X .FIG. 55.Longitudinal section through a gill-book of a newly-hatchedscorpion, x ^-jp.FIG. 56.Longitudinal section through the central eye of an embryo a shorttime before hatching, showing the closure of the cerebro-optic invaginationand the three layers of the eye. X - ^ .FIG. 57.Longitudinal somewhat oblique section through the central eyesof a newly-hatched scorpion. X *$&.FIG. 58.A few cells of the same eye more highly magnified, and showingthe inpushing in the vitreous layer.FIGS. 58, a, b, c.Transverse sections through the same eye at differentlevels.FIG. 59.Section through the la teral eyes of a newly-hatched scorpion.X if*.FIG. 60.Transverse section through the posterior part of the body of anewly-hatched scorpion, showing the fully-formed intestine, the Malpighiantubes, the nerve-cord, and the trabecular tissue fillin g up the ccelomic space.

X * * * .FIG. 61.Transverse section through the intestine further forward, whereit is not yet properly formed, showing the irregular hypoblast cells and yolk-spheres. X if*.


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