nucleolar behaviour in the mitosis of plant cells.nucleolar behaviour in the mitosis of plant cells....

Nucleolar behaviour in the Mitosis of Plant Cells.By

Priscilla E. Frew and Robert H. Bowen,

Department of Zoology, Columbia University.

With Plate 11.

INTRODUCTION.

THE process of mitosis is one of the most impressive and in-triguing of biological phenomena. Since its discovery it hasbeen responsible for an increasing amount of speculation as toits nature and exact significance. With the development of thechromosomal theory of heredity the intent of so elaborate aprocedure has become clear, but we are still in almost completeignorance of the dynamic factors involved in most of its phases.Theories as to the exact nature of these forces are very numerous,but no one of them seems adequate to explain more than alimited number of the facts.

It has become clearly recognized from a mass of observationand experiment that the apparent continuity of the whole pro-cess of mitosis is the result of a remarkable synchronization ofa number of different phases, each one of which must probablybe explained independently. One of these phases that is largelyindependent of the others is the anaphasic movement of thechromosomes. This paper is chiefly concerned with observa-tions1 that seem to have some significance in relation to theoriesof the factors involved in bringing about the poleward move-ment of chromosomes.

1 The major facts described in this paper were first tentatively made outduring the course of an extended study of plant material with anotherpurpose in view. A subsequent prolonged search through the literature ofplant cytology has brought to light a considerable number of apparentlysimilar cases among angiosperms, in none of which, however, have thephenomena been very completely described.

198 PRISCILLA FREW AND ROBERT H. BOWEN

MATERIAL AND METHODS.

The material for this study comprised members of the genusC u c u r b i t a.1 Root-tips of the Hubbard squash variety ofC u c u r b i t a m a x i m a , and of the Connecticut Field, WinterLuxury, and English Vegetable Marrow varieties of the pumpkin,C u c u r b i t a p e p o , were prepared for study. The seeds weresprouted on wet filter-paper in moist chambers, and the root-tipswere cut off with a razor and fixed, when 2-3 mm. in length.

The following fixatives were used : Mottier, Bouin, Hermann,strong and weak Flemming, vom Eath, and a saturated aqueoussolution of corrosive sublimate. Fixation in Mottier's fluid wasfollowed by the regular Benda method of mordanting andstaining. Auerbach's acid fuchsin-methyl green combinationwas used on sections of root-tips that had been fixed in thesublimate solution. After all the other fixing fluids either Fe-haematoxylin or safranin was used as a basic dye. This wassometimes followed by counterstaining with light green oreosin. Both longitudinal and cross-sections were cut at thick-nesses of 4r-(j micra.

OBSERVATIONS.

A. The R e s t i n g N u c l e u s .The cells of the meristematic region of the root-tip of both

species of C u c u r b i t a studied have relatively large nuclei, thegreater part of the contents of which consists of a relativelyhuge,'spherical nucleolus2 (fig-. 1, PI. 11). In the sectioned andstained material this nucleolus is commonly surrounded by aclear, unstained area, which probably is a shrinkage artifact.The nucleolus stains heavily with haematoxylin and safranin.With Auerbach's acid fuchsin-methyl green combination it isstained red ; with Benda it is a brick-red colour. Outside the

1 We have from time to time also noted stages indicative of phenomenasimilar to those here to be described in Cucu rb i t a in several otherangiosperms (and, with some modifications of detail, in E q u i s e t u m ) .

2 In root-tip cells of many plants the occurrence of an unusually largeamount of nucleolar material is a striking characteristic. This may beconcentrated in one very large nucleolus, as in C u c u r b i t a , or distributedin several distinct masses, as in E q u i s e t u m , &c.

NUCLEOLAE BEHAVIOUR IN PLANTS 199

nucleolus there is a small region containing scattered chromatingranules. The identification of this material as chromatin isbased particularly upon the fact that it is stained green by theAuerbach combination. The granules are very small, and it wasnot possible to determine their exact relation to a general nuclearreticulum. Except for these granules the region between the nu-cleolus and the nuclear membrane appears rather homogeneous.

B. The Pro p h a s e .

In C u c u r b i t a m a x i m a , prior to the breakdown of thenuclear membrane in the late prophase, there is formed a coarsespireme of fairly uniform diameter (fig. 2, PI. 11). The relationof this spireme to the scattered chromatin granules seen in theresting nucleus is obscure, the small size of the cells making itdifficult to distinguish early stages in the spireme development.In C u c u r b i t a pepo we have been unable to find a similarspireme. The formation of the spireme in C u c u r b i t am a x i m a appears to be accompanied by no marked change inthe nucleolus beyond a possible slight decrease in size. Eithercontemporary with, or prior to, the disappearance of the nuclearmembrane the spireme apparently becomes broken up into anumber of small, spherical or cylindrical chromosomes, clus-tered on the periphery of the nucleolus, which is now irregular inshape (fig. 3, PI. 11). By the time the spindle has developed andthe chromosomes have moved to their equatorial position uponit the nucleolar material has in many cases entirely disappeared,as, indeed, is customary in mitosis in both animal and plantcells. In some cases,1 however, the nucleolus behaves as thoughits material was relatively too abundant for dissolution to becompleted during the prophase period.2 It then persiststhroughout the entire prophase period and is caught in thecentre of the spindle-area, becoming elongated to form acylinder with its long axis parallel to that of the developingspindle (fig. 4, PI. 11). This alteration in shape is significantin view of the metaphase phenomena to be described below.

1 Particularly in the outer layers of the plerome.2 See also Tischler (1922).

200 PRISCILLA FREW AND ROBERT H. BO WEN

C. The M e t a p h a s e .

Nucleolar material thus frequently persists up to the begin-ning of the metaphase. In such cases, in the early metaphasethe nucleolus, in the form roughly of a cylinder, is always foundin the centre of the equatorial plate of chromosomes, with itslong axis parallel to that of the spindle (figs. 5, 10, and 11,PI. 11). Polar views (figs. 10 and 11, PI. 11) emphasize the factthat the nucleolus is actually within the spindle region and sur-rounded by a broad ring of chromosomes. Perhaps a betterdescription would be that the nucleolus ' perforates ' the plateof chromosomes more or less centrally, being often much con-tracted at the point of perforation (fig. 10, PI. 11), and enlargedat both extremities. This orientation is not always symmetricalwith respect to the equatorial plate, since it frequently happensthat the chromosomes surround the nucleolus in a plane muchcloser to one end of the cylindrical nucleolus than the other.

The nucleolus next becomes drawn out to a shape usuallyresembling a dumb-bell (figs. 6, 7, and 8, PI. 11). Occasionallynearly all of the cylinder lies in one-half of the spindle, and insuch a case the nucleolus becomes drawn out to a shape shownin fig. 9, PI. 11. In either case, the two ends of the nucleoluscontinue their movement towards opposite spindle-poles witha consequent stretching of the part in the equatorial region, untilfinally the strand connecting the two polar portions is ruptured(figs. 13 and 14, PI. 11). The two masses of nucleolar materialthus formed continue their poleward migration and eventuallyreach the poles of the spindle (figs. 12, 15, and 16, PL 11). Aswould be expected from the variation in orientation of thenucleolus before and during its division, the two spheres maybe nearly equal in size (fig. 12, PI. 11), or markedly unequal asin fig. 15, PI. 11. Sometimes the size differences are even morepronounced. When the nucleolar fragments are approximatelyequal, as in fig. 16, PI. 11, their simulation of the centrioles insome animal-cells is rather striking. During all this time thenucleolus appears to be undergoing a progressive shrinkage insize, although positive determination of this fact is difficult

NUCLEOLAR BEHAVIOUR IN PLANTS 201

because of initial variations in nucleolar size at the close of theprophase. However, we infer that there is a shrinkage from thefact that the sum of the volumes of the largest pair of spheresobserved in the polar position never appears to be as great asthat of the larger nucleoli before metaphase. During the meta-phase phenomena the nucleolus is commonly seen in fixed andstained preparations to be surrounded by a narrow clear zone.One is tempted to assume that this represents a region wheredissolution of the nucleolus, resulting in a reduction of its size,is taking place, but possibly it is an artifact resulting fromshrinkage, like that seen in the resting nucleus.

D. The A n a p h a s e .

As is usually the case, the anaphase migration of the chromo-somes in C u c u r b i t a appears to be a very rapid process, andintermediate stages of it are very rare. Apparently no significantchange takes place in the nucleolar masses, and, being slightlypushed away from their exact polar locations, they are seen atthe end of the anaphase lying adjacent to the chromosome platesand appearing very much as at the close of their metaphasemigration (fig. 17, PL 11).

E. The T e l o p h a s e .

During the telophase reconstruction of the nucleus thenucleolar masses continue to remain clearly apart from thechromosome groups, although they may be in their near vicinity(figs. 18 and 19, PL 11). The shrinkage process apparentlycontinues (fig. 19, PL 11). During the nuclear reconstructiona new mass of nucleolar material appears in the midst of eachgroup of chromosomes. It has no apparent connexion with theold nucleolar mass although the occasional close juxtapositionof the chromosome plates and these old nucleolar fragmentssometimes makes this point difficult to determine. But incases where the polar movement has proceeded to such anextent as in fig. 17, PL 11, it would seem practically certain thatan intimate topographical relationship could never occur with-out a further movement of either the chromosomes or the


nucleolar mass, and for such movements there is no evidencewhatsoever. All the facts indicate that during the reorganiza-tion of the daughter nuclei the nucleolar fragments of the parentcell remain entirely extranuclear, and ultimately undergo com-plete disintegration in the polar cytoplasm. In sister cells inwhich the nuclei have been completely reconstructed it has notbeen possible to identify any nucleolar material in the cyto-plasm. Small granules are occasionally seen, but they cannotcertainly be distinguished from cytoplasmic granules that neverhad any relation to the nucleolus. Probably by this stage thedissolution of the nucleolar remnants has proceeded to such anextent that they have either disappeared entirely or shrunkento insignificant and unrecognizably small masses. We wish tobe very emphatic upon one point, namely, that, in spite of thesimulation of chromosome behaviour shown by the equatorialorientation and subsequent division of the nucleolus, the evi-dence practically demonstrates that this is merely a necessaryresult of the more or less accidental catching of the nucleolus inthe spindle-region. The nucleolus is strikingly different fromthe chromosomes in that there is no direct continuity of itssubstance from one cell-generation to another.

DISCUSSION.

In this study our attention has been chiefly directed towardthe nucleolar phenomena, and observations of other structuresin the cell have been entirely incidental. This discussion will belargely confined to the same topic, namely, the behaviour of thenucleolus during mitotic division.

A resting nucleus of the same general character as that inG u c u r b i t a has been described in Azol la by de Litardiere(1921). The similarity centres largely in the presence ofnumerous small granules of chromatin of fairly uniform size.In the case of Azol la the evidence strongly suggests that thesegranules are chromosomes which are persistent throughout theinterkinetic phase. These chromosomes appear to go on themetaphase plate without ever forming a definite spireme. Ourobservations indicate the possibility of similar phenomena in


C u c u r b i t a pepo , but positive determination of this factawaits more careful investigation with this particular objectin mind.

The nucleolus is usually described as approximately sphericalin resting plant-nuclei, and the clear space around it is commonlyshown in published figures. Our tentative conclusion that thisclear area is largely an artifact—the result of shrinkage—isbased in part on the statement of Lundegardh (1912) that inliving cells such a space is very seldom observed, even in thoseforms which show it most clearly in the resting nuclei of fixedpreparations.

The irregularity in shape of the prophase nucleolus has alsobeen frequently described. Nucleoli have been observed toundergo amoeboid changes of form during the prophase in livingcells of Char a (Zacharias, 1902) and Vicia (Lundegardh,1912). We have no clue to any broad significance which mayattach to this phenomenon and it probably merely indicatessome change in the physical character of the nucleolus or nuclearsap that is incidental to the prophase condition in the nucleus asa whole. The spherical form of the resting nucleolus is assumedby Lundegardh to indicate a very fluid consistency. If thisbe true, the prophasic amoeboid form might be construed asan indication either of an increase in viscosity or of a decreasein surface tension.

During the prophase of cell division the nucleolus commonlyundergoes a progressive shrinkage, Avhich has often been em-phasized because of its supposed indication of the transfer ofmaterial from the nucleolus to the growing chromosomes. Thisshrinkage commonly results in the disappearance of the nucleo-lar material in the earlier prophase, or at the latest before thebreakdown of the nuclear membrane, as in A Hi urn, &c. Inother cases, however, the nucleolar substance may persist intothe late prophase and thus be cast out into the cytoplasm atthe breakdown of the nuclear membrane, as in P u s t u l a r i a(Bagchee, 1925), where the final stages in nucleolar dissolutiontake place in the cytoplasm adjacent to the equatorial region ofthe spindle. Finally, in some cases, the persistent nucleolus

204 PBISCILLA FREW AND ROBERT H. BOWEN

may be caught in the spindle and involved in the actual divisionprocesses, as in C u c u r b i t a. The fact that the decrease ofnucleolar mass progresses throughout the anaphase and telo-phase stages in the last-mentioned cases seems to us significantin view of the hypothesis occasionally put forward that theprophase shrinkage of the nucleolus indicates some direct con-tribution from the nucleolus to the concomitant increase in thechromatin. Certainly chromosome growth usually ceases in thelate prophase, yet the nucleolar shrinkage continues, whilethe nucleolar remnants become widely separated from the chro-mosomes (fig. 14, PI. 11). This whole series of events seems toindicate that we have to do here with two coincident but other-wise unrelated processes, namely, increase of chromatin anddecrease of nucleolar substance. The nucleolus disintegrates,not because it contributes to chromatin development, but moreprobably as a result of the new physical or chemical conditionsin the nuclear sap—conditions that are probably incidental tothe mitotic process as a whole.

The division of what superficially at least resembles nucleolarmaterial,1 and the polar migration of the division productscoincident with or subsequent to the actual separation of thechromosomes has been described in a variety of forms. Anexample of such a process in the Protista is the case ofEuglena(Keuten, 1895 ; Tschenzoff, 1916, and others). It has beendescribed among the filamentous green Algae by Berghs(1906) in S p i r o g y r a , by Nemec (1910a) in C l a d o p h o r as i m p l i c i o r , and by de T'Serclaes (1922) in C l a d o p h o r ag l o m e r a t a . It has been noted also in the myxomyceteS p o n g i o s p o r a by Osborn (1911), and in the Plasmodio-phorales by Cook (1928). One example has been reportedin the pteridophytes—the case of Mars i l i a (Berghs, 1907)where the nucleolus is caught in the spindle, but dissolvesduring the metaphase. Many cases of this sort have also beenmore or less completely reported in the higher plants. It

1 Although the general opinion seems to be that in lower forms thismaterial is not actually equivalent to the true nuoleolus of the higherplant-cell.


has been unmistakably described in P h a s e o l u s by Rosen(1896), in E o r i p a by Nemec (1897), in So l anum byNemec (1899), in Alnus and H i b i s c u s by Nemec (1901aand b), in P h a s e o l u s by Wager (1904) and Martins Mano(1905), in E i c i n u s and C u c u r b i t a m a x i m a by Nemec(1910b), in C u c u r b i t a pepo by Lundegardh (1912), inL u p i n u s by de Smet. (1914), in H e l i a n t h u s by Tahara(1915), in Olivia by Van Camp (1924), and in Canna andL u p i n u s by Schaede (1928).1 In all these cases in the higherplants it appears that nucleolar material sometimes persistsuntil the metaphase, in which case it is caught in the spindle,and is probably divided essentially as we have described inC u c u r b i t a . In the lower plants the division of nucleolarmaterial usually accompanies the separation of the chromo-somes instead of preceding it, and not infrequently the nucleolarderivatives are ultimately incorporated in the daughter nucleias the definitive nucleolus. In most of the cases mentionedabove among angiosperms the original descriptions are limitedto a few sentences and but two or three figures. The work ofVan Camp (1924) is distinguished by his more positive identifica-tion of the nucleolar material by means of staining with theAuerbach combination. Both Lundegardh's and Nemec'sdescriptions of C u c u r b i t a are limited to a few figures andcasual references in the text, since the main purpose of theirstudies lay in another direction. Van Camp's paper is fairlycomplete and includes some review of earlier work.

Apparently Nemec at least clearly appreciated the significanceof these observations in relation to general theories of thefactors responsible for the anaphase movement of the chromo-somes. At the time of his paper (1901) the most widely acceptedhypothesis was that of Van Beneden and others that thechromosomes were pulled to the poles by the contraction of the

1 There are also cases in which the nucleus characteristically containsmore than one nucleolus, the nucleoli being transported intact to theopposite spindle poles during the prophase-metaphase transition—forexample, Karsten (1893) in P s i l o t u m , and Bargagli-Petrucci (1905) inE q u i s e t u m . See also Zimmermann (1893) and Lenoir (1926).

206 PEISCILLA FREW AND ROBERT H. BOWEN

attached spindle fibres. In a rather extensive discussion of thisview Xemec strongly emphasized the point that the division andpolar migration of a nucleolus, to which there were certainlyno spindle fibres attached, was a practically insurmountabledifficulty in the way of explaining anaphase chromosome move-ment as due to fibrillar contractility. This theory, Nemecpointed out, could only be maintained if it assumed that thechromosomes and nucleolar fragments were moved to the polesby different forces.

This same objection to theories of fibrillar contractility waslater indicated by Bonnet (1912). This author was more em-phatic in his declaration that the division of the nucleolusdefinitely ruled out the theory of Van Beneden, and he concludedthat all that is known of the process of anaphase migration isthat there exist in the cell at mitosis certain factors whichordinarily cause the movement of certain cellular constituentstowards the poles. He believed that these forces were notnecessarily confined to the spindle region, since amyloplasts inthe surrounding cytoplasm occasionally participated in the pole-ward movement. Bonnet was completely sceptical about therole of a spindle in the process, and believed that it had merelya remarkable chronological relation to the other phenomena—anextreme view to which we can hardly subscribe when we con-sider the universal presence of a spindle in cells undergoingmitosis.

In an instructive discussion of the mechanism of mitosisTischler (1922) reiterates this argument against the fibrillarcontractility theory. This author also considers its relation tothe so-called ' Stemmtheorie ' originated by Driiner (1895) andrecently revived by Belar (1927). This theory holds that thechromosomes move to the poles because they are pushed apartby the growth of the interzonal fibres as a whole (Stemna-korper) in the region between the daughter chromosomes. AsTischler points out, this theory is quite adequate to explain thepolar migration of all the chromosomes simultaneously, butcompletely fails to allow for a p r ecoc ious division of nucleo-lar material. It might be added that the theory also fails to


explain the precocious anaphase migration of some sex chromo-somes, and even of some autosomes. These difficulties haverecently been considered by Belar (1928), who suggests meansof circumventing them.

It is evident that our observations on C u c u r b i t a offer noimmediate solution of the problems presented by the mitoticspindle and the anaphase movements of the chromosomes. Theydo, however, clearly emphasize two features of importance.Firstly, that the anaphase migration of the daughter chromosomesis apparently not directly due to the so-called spindle-fibreswith which they are in connexion, since the nucleolar portionsbehave in exactly the same way without any spindle-fibreattachments whatever; and secondly, that the products ofthe division of the nucleolus move to the spindle poles while thechromosomes, though already divided, remain unmoved in theequatorial region of the spindle.

With regard to the first of these points, our observationswould seem to suggest that the spindle area represents a regionin which are localized those forces of whatever kind which areresponsible for anaphasic movements. This localization is moststrikingly demonstrated in cases like that of P u s t u l a r i a ,where the nucleolus is in most instances left outside the spindleat metaphase and disintegrates in the equatorial cytoplasm, buton rare occasions divides as in C u c u r b i t a simply because itis by chance caught in the spindle area. It is clear from otherwork that movements are also on foot in the general cytoplasmlooking toward the bipolar orientation of chondriosomes andarchiplasts during mitosis, but the identity of these with thefactors at work within the spindle area seems to us by no meansso clear as Bonnet, for example, assumed.

If the spindle then represents a specialized area within whichvery definite, directed movements take place, it seems probablethat whatever the force at work, it would be equally potentregardless of the nature of the bodies which found themselvesin the spindle region—whether chromosomes or nucleoli. Thus,in the present case, the circumstances strongly suggest that thepresence of the nucleolus within the spindle region is a pure


matter of chance, dependent primarily on the fact that itsdisintegration during the prophase has proceeded too slowly.The nucleolus may thus be thought of in terms of some foreignbody inserted into the spindle area, and its resulting divisionand movements depend entirely on just where the nucleolushappens to lie with respect to the mid-region of the spindle.Presumably any other small mass of proper consistency insertedinto the spindle would behave in the same way.1 All these thingsindicate that the ' something ' which moves chromosomes to-wards the spindle poles is not especially associated with thechromosomes as such. Neither does this ' something ' reside inthe chromosomes themselves. Bather do the chromosomes movein response to the same forces which would move anythingplaced in the same position.

It is, then, abundantly clear that the movements of thechromosomes cannot depend on the ' contraction ' of spindle-fibres, or any other such special apparatus. Some hypothesisinvolving protoplasmic streaming could doubtless be suggested,and, indeed, the recent work of Chambers (1917) on protoplasmiccurrents in the asters of the cleaving egg,2 and the studies ofSpek (1918) on artificial simulacra of cleavage processes, recallthe old suggestion of Biitschli (1876) that chromosome migrationmay be effected by streaming movements in the spindle ; alsothe view later held by Berthold (1886) that the chromosomesmight be pushed apart by the streaming of cytoplasmic materialinto the mid-plane of the spindle. Unfortunately we haveabsolutely no definite information concerning the dynamic con-

1 In this connexion it is of unusual interest to recall the observations ofKonopacki (1911) on the behaviour of the nucleus in sea-urchin eggsexposed to hypertonie solutions. The abnormal chromosomes, sometimeseven the i n t a c t nucleus, having reached a position between the asters,are dragged apart into two portions which eventually reach the spindlepoles. The figures given by Konopacki of the division of intact nuclei bysuch a procedure are remarkably suggestive of the behaviour of thenucleolus in Cucu rb i t a .

- Most interesting is the fact that oil droplets on the periphery of anaster will be carried towards the astral centre if they be first guided bya microdissection needle into the inflowing stream (Chambers, 1917).

NUCLEOLAK BEHAVIOUR IN PLANTS 209

ditions which prevail in the spindle, and the presence of anythingcomparable to ' streaming movement' has not been recognized.It seems to us, therefore, at present of doubtful usefulness toattempt to trace the behaviour of bodies in the spindle area tomere protoplasmic currents. The important point is that what-ever the nature of the force at work in the spindle area, it isnothing which has to do specifically with the chromosomes. Itis rather part and parcel of the whole achromatic division figure.

The difficulty remains that the chromosomes do not moveuntil a definite moment in the mitotic cycle has been reached,in spite of the clear evidence that the factors responsible fortheir migration are operative relatively early in spindle forma-tion. The reasons for this stability of the equatorial chromo-some complex seem to be bound up with the well-known factthat the spindle area is, in part at least, a region of higherviscosity than the surrounding cytoplasm. This is true not onlyof the metaphase, but particularly of the late anaphase, asBelai's (1927) recent study of living cells has so clearly demon-strated. But concerning any details of the morphologicalstructure of this viscous spindle area, again we must confess analmost complete ignorance. It is, however, difficult to see howthe viscosity of the spindle can be directly invoked as an ex-. planation for the retention of the chromosomes in an equatorialposition, since the nucleolar fragments are meanwhile movingpoleward presumably propelled by the same mechanism whichfinally moves the chromosomes as well. In this connexion it isperhaps of interest to recall the demonstration recently givenby Nassonov (1918) that in plant mitosis the ' fibres ' attachedto the chromosomes are definitely demonstrable by osmic-acidimpregnation, while the so-called central spindle remainsunblackened and apparently structureless.1 If the ' fibres 'thus demonstrated represent regions of unusually high proto-plasmic viscosity, the metaphase retention of the chromosomes

1 Nassonov's extended discussion of the structure of the spindle is mostinteresting in the light of the suggestions made in this paper. His accountof the poleward retraction of the chromosomal fibres is particularly note-worthy.

NO. 290 P


in the equator would receive an obvious explanation. Theultimate movement of the chromosomes could be brought aboutby the progressive liquefaction of the fibres at one or both ends.The interzonal fibres, or ' Stemmkorper ', developed betweenthe diverging chromosomes, represent presumably the forma-tion of a new area of very high viscosity which leaves the spindlein a remarkably firm condition and perhaps in its developmentassists in the extreme pushing apart of the daughter chromo-some groups as a whole—a phenomenon which is indeed wellknown from the work of several observers, most recentlyBelaf (1927).

These suggestions are here put forward by way of furtherapproach to a problem which has hitherto proved extra-ordinarily baffiing. The ultimate solution of the structureand dynamics of the ' spindle ' clearly demands much furtherinvestigation.

SUMMARY.

1. The resting nuclei of C u c u r b i t a pepo and Cucur-b i t a max ima contain a single, spherical nucleolus that isrelatively very large. Part, at least, of this nucleolus persistsduring nearly the entire process of mitosis. During the nieta-phase it lies in the equatorial plate and becomes elongated in thedirection of the long axis of the spindle, to form first a cylinderand then, with further elongation, a dumb-bell-shaped structure,which finally separates into two fragments that migrate to theopposite poles of the spindle. This entire movement occursprior to any anaphase migration of the chromosomes. Thenucleolar fragments apparently are not included in the daughternuclei during the telophasic reconstruction, but degenerate inthe cytoplasm.

2. Examination of the literature suggests that a similarprocess of nucleolar division probably occurs in a wide varietyof plant-cells.

3. The bearing of this type of nucleolar division on theoriesof the dynamics of anaphase migration of the chromosomes isbriefly discussed.


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P2


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EXPLANATION OF PLATE 11.

All of the figures have been outlined as far as possible withthe camera lucida at an initial enlargement of approximately1,675 diameters, and subsequently completed free hand. In

nucleolar behaviour in the mitosis of plant cells.nucleolar behaviour in the mitosis of plant cells....

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