POSTGRAD. MED. J. (1962), 38, 193

THE FOETAL MEMBRANESA Review of the Anatomy of Normal Amnion and Chorion

and Some Aspects of Their FunctionGORDON BOURNE, F.R.C.S., M.R.C.O.G.

Obstetric and Gyncecological Surgeon, St. Bartholomew's Hospital, London, E.C.I

THE foetal membranes of the human are composedof the entire amnion and the non-placentalchorion. The amnion is the innermost of the twohuman fcetal membranes and, as such, is in contactwith the contents of the amniotic sac, namely theamniotic fluid, the foetus and the umbilical cord.The chorionic membrane, which is attached tothe outer surface of the amniotic membrane,separates the amnion from the decidua and thematernal uterus.Throughout pregnancy the human foetus is

surrounded by amniotic fluid which is enclosedwithin the sac formed by the faetal membranes.It is an important fact that both the amnion andchorion are composed entirely of foetal tissue. Theclose proximity of these structures to the maternalorganism, as they line the inner surface of theuterus, may influence their structure or behaviour,but their chromosomal sex remains unaltered bythis close association (Bourne, 1962; Klinger,I957). It is now accepted that the fluid environ-ment of the foetus should be regarded as a mediumwhich is constantly being renewed or replaced,and perhaps, altered in composition. It is not,therefore, a static liquid but, similar to the otherfluid compartments of the body, the liquor amniihas a circulation for which the term 'AmnioticFluid Circulation' has been evolved. According toPlentl (I957), from work carried out by the use ofdeuterium, the exchange of water is about 450ml. per hour at term and there are differentialrates of transfer of sodium and potassium ions.The exact sites of origin of amniotic fluid are un-known nor is there any definite evidence to showhow such large amounts of fluid are conveyedfrom the amniotic sac to the mother or to accountfor the passage of various ions at different rates.Undoubtedly some of the fluid passes into thefoetus and thence to the mother via the fcetalcirculation and placenta, but it is very probable thatsome of the amniotic fluid exchange occurs viathe foetal membranes into either the placenta itselfor the uterus direct.

Theoretically, the foetus is in complete chargeof its own environment since any fluid or sub-

stance that enters or leaves the amniotic compart-ment must pass through tissue of foetal origin. Anunderstanding of the anatomy, physiology andpathology of the foetal membranes has thereforebecome important if pregnancy and its compli-cations are to be better understood.

The Structure of the AmnionIt is now generally accepted that the human

amnion is not merely an epithelial lining for theuterine contents, but that it is a complicated tissueconstructed histologically of several differentlayers. Careful study of the amnion reveals thatit is composed of five layers (Bourne, 1960).These are shown in a semi-diagrammatic mannerin Fig. i. The face of this illustration shows thelayers as they appear in cut vertical sections,whilst the treads of the steps show how they areseen when membrane preparations of the tissueare examined. Confusion has arisen over the useof ' inner' and ' outer' in relation to the amnionand chorion; the term ' inner ' refers to that part,or layer, nearest to the amniotic cavity and theterm ' outer' refers to that part, or layer, nearestto the myometrium.The amnion, which is normally o.oz to 0.5

cm. in thickness, consists of five layers. These are,from within outwards:-

I. Epithelium.2. Basement Membrane.3. Compact Layer.4. Fibroblast Layer.5. Spongy Layer.I. The Epthelium is the innermost layer and is

in contact with the amniotic fluid. It consists of asingle layer of cells which are usually cuboidalbut may be columnar over the placenta or flattenedinto pavement cells over isolated areas of theremainder of the amnion. The apex or innersurface of the cells is slightly convex in shape.Small evaginations of the cell membrane protrudeinto the amniotic fluid from the free surface asmicrovilli to form a brush border (Fig. 2). Thewell-defined intercellular membrane is con-densed at the apex to form terminal bars.

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194 POSTGRADUATE MEDICAL JOURNAL April 1962

~orp~ Layer

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FIG. i.-Composite diagram to illustrate the appearances of the layers ofreflected amnion and chorion when examined by both membranepreparations and routine cut sections.

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FIG. 2.-Amnion Epithelium. The upper, or free,margin of a portion of two columnar-shapedepithelial cells from the placental amnion atterm. Microvilli (M.V.) cover the surface ofthe cells. The nuclei (N) are irregular inoutline. Inter-cellular canals (I.C.C.) ex-tending from the free surface are dilated inplaces to form vacuolar-like spaces. Manysmaller vacuoles are present in the cell cyto-plasm. x 6,500.

The base or outer edge of the cells is in contactwith the basement membrane. At high magni-fication the basal region of the cell is complexand irregular in outline forming irregularly-shaped basal processes of various sizes (Fig. 3).These basal processes are in intimate contact withthe basement membrane to which they are denselyadherent.Along the lateral aspect, the two membranes of

adjacent cells delimit a series of irregularly shapedvacuoles (Fig. 2). Some of these vacuoles simplyappear to extend into adjacent ones; others areconnected by fine channels formed by the closeapposition of the two cell membranes. Whilst themajority of these vacuoles are located in theregions between adjacent cells (that is, hollowedout into the side of the cells) a few are seen lying

deeper in the cytoplasm. Projecting into some ofthe vacuoles are a variable number of microvilli,generally less per unit area than those seen onthe upper surface of the cells.When viewed from above, as in membrane

preparations, the cells are seen to be polygonal inshape, appearing as an irregularly arrangedmosaic.The epithelial cells normally contain a single

nucleus which is irregular in outline and sometimesfenestrated. The cytoplasm at high magnifi-fication is dense and granular and-contains manyvacuoles which vary both in size and content(Fig. 2). Mitochondria are scarce and small. AGolgi apparatus has been described by someworkers but has not been seen by others. Studiesof the amniotic epithelial cells at high resolution

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April I962 BOURNE: The Fcetal Membranes 195

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FIG. 3.-Amnion Epithelium. The baseof an epithelial cell (E.C.) showingthe basal processes which form acomplex arrangement with the base-ment membrane (B.M.). Intercellularcanals (I.C.C.) extend upwards be-tween the cells. The compact layer(C.L.) of the amnion lies at thebottom of the photograph. X 8,500.

and magnification demonstrate that comparativelylarge areas of the cytoplasm contain numerouspaired parallel lines and circular elements (Fig.4) (Bourne and Lacy, I960). These observationsare interpreted as showing the presence of mem-branes arranged mainly in the form of fine canalswith some occurring as parallel sheets. Themembranes are each about 60 A thick, the dis-tance between then (canal lumen) is about Ioo A.Examination of representative parts of the cellsurface, where it borders the lateral and basalsystem of vacuoles referred to above, reveals manycircular apertures (about 75-Ioo A internaldiameter) which communicate with the intra-cellular channels and canals. The evidencesuggests, therefore, that amniotic epithelial cellscontain an extensive system of canals and channelswhich communicate directly with the extra-cellular ' space '.

2. The Basement Membrane. This is a thinlayer composed of a network of reticular fibresand is well marked over both the placental and thereflected parts of the amnion. The superficial orinner aspect of this layer has a complex relation-ship with the epithelial cells. Short, blunt pro-cesses from the bases of the epithelial cells inter-digitate with similar processes that arise from thebasement membrane.

3. The Compact Layer. This relatively dense

layer is almost completely devoid of cells andconsists of a complex network of reticular fibres.The outline of the mesh alters as tension isapplied to it. This layer, which is probably thestrongest of the amniotic layers, is rarely thickenedby cedema, and it appears to resist, to some extent,penetration by leucocytes when the amnion isinflamed.

4. The Fibroblast Layer. This is the thickestlayer of the amnion. It is composed of a loosefibroblast network embedded in a mass of reticulin.The cells occasionally show phagocytic activity.

5. The Spongy Layer. The tissue of the extra-embryonic coelom is compressed between theamnion and the chorion to form the spongylayer. Its wavy bundles of reticulin, bathed inmucin, render routine staining difficult, but thesebundles are seen by phase microscopy as branch-ing fibres having triangular shaped nodes at thejunctions. A few isolated fibroblasts are presentin this layer. It frequently becomes edematousand, as such, accounts for the increase in thicknesswhich often occurs in the amnion. This layerpermits the amnion to slide upon the underlyingchorion which is firmly adherent to the maternaldecidua (Fig. 5).Macrophages (Hofbauer Cells). These were

described in the placenta by Hofbauer in 1905,since when they have frequently been known as

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i96 POSTGRADUATE MEDICAL JOURNAL April I962

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i-· FIG. 4.-Micrograph of the num-erous membranes found invarious regions of the cytoplasmof amniotic epithelial cells. Themembranes are arranged mainlyin the form of canals whichappear to open on the cellsurface and communicate withthe extracellular space.X 100,000.

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FIG. 5.-Human amnion and cho-rion. A vertical histologicalsection through amnion (above)and chorion (below). Comparewith Fig. i, A = epithelium,C = compact layer, D = fibro-blast layer, E = spongy layer,G = reticular layer, I=trophoblast, OV = obliteratedvillus. Note the fine reticulartissue connecting the spongylayer with its adjacentstructures. Hgematoxylin-eosinx 90.

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BOURNE: The FIetal Membranes

Hofbauer cells, though they had been previouslydescribed and are probably the cells discussed byMuller in I847. They are single nucleated macro-phages varying from 15 to 35 ,J in diameter andhaving a foamy or vacuolated cytoplasm. Theirsingle nucleus is eccentrically situated and oftenreniform in shape. The cellular outline varies,being usually oval or round, but, not infrequently,pseudopodia are present. Hofbauer cells arenormally present in the fibroblast and spongylayers of the amnion and in the cellular, reticularand trophoblast layers of the chorion. They areoccasionally seen in the compact layer of theamnion, especially if they are actively phagocytic.In vitro, these cells readily take up dye which theyconcentrate in their vacuoles and in vivo theycommonly contain relatively large quantities ofmeconium.

The ChorionThe chorion being the outermost of the two

foetal membranes is in contact with the amnion onits inner aspect and the maternal decidua on itsouter aspect. The placenta is composed ofchorion and is formed by the hypertrophied villiof the chorion frondosum. The chorionic villiover the remainder of the chorion (chorion laeve)atrophy and may be recognized in histologicalsections as obliterated or ghost villi (Fig. 5). Thenon-placental chorion consists of four layers.These are, from within outward:

6. Cellular Layer.7. Reticular Layer.8. Pseudo-basement Membrane.9. Trophoblast.6. Cellular Layer. This is a thin layer consisting

of an interlacing fibroblast network. It is fre-quently imperfect or completely absent from thechorion when examined at term, but is moreeasily recognized in early pregnancy.

7. Reticular Layer. This forms the majorityof the thickness of the chorion and consists of areticular network, the fibres of which tend to beparallel. Nodes are present on the fibres at thoseplaces where branching occurs. A few fibroblastsare present, together with many Hofbauer cells.

8. Pseudo-basement Membrane. This forms abasement membrane for the trophoblast. It is alayer of dense argyrophil connective tissue that isfirmly adherent to the reticular layer above andwhich sends anchoring and branching fibresdown into the trophoblast.

9. Trophoblast. The deepest layer of thechorion consists of from two to io layers of tropho-blast cells in contact, on their deeper aspect, withmaternal decidua. This layer contains the obli-terated chorionic villi, one of which is showndiagrammatically in Fig. i.

Blood SupplyThe only blood vessels present in the foetal

membranes are those in the region of the placentaitself. The umbilical vessels, on reaching theplacenta, spread out and radiate over its surface,travelling within the substance of the reticularlayer of the chorion. Although this layer containsthe foetal vessels, it does not receive a capillaryblood supply from them, either over the placentaor in the reflected part of the membrane. Theamnion has no blood supply of its own, nor does itcontain blood vessels even over the placenta.Amniotic Fluid ExchangeFrom the above description and from the experi-

mental work that has so far been performed itseems reasonable to conclude that the amnion andchorion do influence the fluid environment ofthe foetus. The structure of the amniotic epi-thelium indicates that it is physiologically capableof taking an active part in some of the amnioticfluid exchange mechanism.

So far as can be determined on a purely morpho-logical basis, it seems likely that some of theamniotic fluid which passes from the foetus tothe mother, and the accompanying differentialrate of turnover of various ions, may be broughtabout by the amnion. As a working hypothesis ithas been suggested that the fluid enters theamnion via (i) the opening between adjacent cells,where it passes into the lateral system of vacuolesand (ii) the upper surface, the area of which isgreatly increased by the presence of microvilli.Within the cells the fluid presumably enters thecomplex system of fine canals and channels. Thesecommunicate with the lateral and basal vacuolesand hence with the extracellular space; thisresults in an enormous increase in surface area.Within the vacuoles there is presumably somemixing of any solution which may have enteredvia the two principal pathways. Assuming thatsome ions travel more readily along one routethan the other, then this could offer an explanation(although much over-simplified) of the differentialrate of turnover of different ions. Having passedthrough the epithelial layer the fluid must permeatethe connective tissue layers. Since these layersconsist mainly of networks of fibres there is noobjection to this on morphological grounds.Transfer of MeconiumMeconium is passed by the foetus at times of

stress, especially when it is subjected to anoxia,and the presence of meconium in the amnioticfluid is accepted by most obstetricians as anindication of anoxia or hypoxia, although theexact level at which this occurs is uncertain.Meconium absorbed by the amnion and chorion,

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!~i~'i~?:~i~1ii:.~im~:.~:-~.~:~~~~~~~~~~~~~~*;-~.ii'-~.!,!~,~?,::~~..~..... FIG. 6.-Meconium-stained membrane. Dense in-clusions of dark brownish-green meconium arepresent in the degenerating epithelial layer.Meconium is also present in the cytoplasm of threefibroblasts in the amniotic connective tissue.Formalin fixed paraffin embedded tissue. Verylight eosin stain. x 435.

stains the membranes green. Histological sectionsof such meconium-stained membranes revealsthat meconium may be present in the epitheliumand the Hofbauer cells of the amnion (Fig. 6). Indeeply stained membranes the meconium penetratesnot only the reticular and trophoblast layers ofthe chorion but also into the decidua, where it maybe found in cells adjacent to maternal capillaries(Fig. 7). This passage of meconium, from amnioticcavity to decidua, is further proof of the movementof material from the foetus to the mother across themembranes.

Amnion NodosumThis is a pathological condition of the amnion

in which the fcetal surface of the amniotic mem-brane is studded with multiple greyish nodules.The term 'Amnion Nodosum' was coined byLanding (1950) when he reported eight cases,five of which suffered from a major congenitalabnormality of the renal tract and three, who werestillborn, were not examined by autopsy, althoughtwo suffered from multiple external abnormalities.Recently Jeffcoate and Scott (1959), in a criticalreview of polyhydramnios and oligohydramnios,reported their findings in 14 cases and cameto the conclusion that the lesion is associatedprimarily with a shortage of liquor, rather thandirectly with the renal defect. The author hasseen amnion nodosum five times and on eachoccasion it was associated with oligohydramniosand renal agenesis.That the incidence of amnion nodosum is

much greater than the 43 reported cases would

imply, is indicated by the fact that Jeffcoate andScott collected 14 cases in less than two years. Thepaucity of the recorded material probably resultsfrom inadequate examination of the amnioticmembrane immediately after delivery. It is alsopossible that the small nodules are not seen, orthat their significance is not appreciated, so thatthe secundines are destroyed before the clinicalcondition of renal agenesis is recognized.

Macroscopic Appearance. The lesions are small,circular nodules situated on the foetal surface ofthe amniotic membrane and the epithelium of theumbilical cord. They vary in colour from whiteto dark grey, or red, but are mostly pale grey witha faint yellowish tinge. The junction of theumbilical cord with the placenta is the centre oftheir area of distribution and it is around thispoint that the nodules are most frequently found.Whilst the nodules are commonest over theplacental amnion they also occur on the juxta-placental amnion, although they are found lessfrequently in areas further from the umbilical cord.The nodules vary in size from microscopic to

3 or 4 millimetres in diameter and, especially nearthe placental-cord junction they frequentlycoalesce to form irregularly shaped plaques but,individually, each nodule has a circular outlineof almost geometric precision (Fig. 8). A charac-teristic feature of the nodules is that they can bepicked off the underlying amnion leaving a semi-translucent saucer-shaped depression. Thenodules are composed of a soft, waxy but granularmaterial which is friable and may be crushedeasily.

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April 1962 BOURNE: The Foetal Membranes 1

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FIG. 7.-Meconium-stained chorion. The reticularlayer of the chorion (R) above and the trophoblast(T) and decidua (D) below. The shrunken anddisintegrating stromal cells lie within empty cleftsin the reticular tissue. The small round dark in-clusions are meconium (M). Note how themeconium has penetrated the membrane and lieswithin the substance of the maternal decidua.Htematoxylin and Eosin x 400.

Microscopic Examination. Many more nodulesare present than can normally be seen by thenaked eye. Histological sections reveal many tinynodes which may be as small as o.I millimetresin diameter. They consist of masses of keratinizedsquamous cells embedded in an amorphousacidophil matrix. The nodes lie directly upon theconnective tissue of the amnion, forming a minutesaucer-shaped depression within the superficiallayers of the amnion. At the site of their attach-ment the amniotic epithelium has, for the mostpart, disappeared though occasionally a few cellsremain. The structure of the nodes in verticalsection gives the appearance of crude and in-efficient whorl formation the centre of which, inthe smaller lesions, is near the centre of thenodule (Fig. 9).

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FIG. 8.-Amnion Nodosum. A portion of amnionnodosum photographed from above and trans-illuminated from below. The nodules are circularexcept where they are distorted by pressure fromneighbouring nodes. Occasionally they coalesce toform oval or irregularly-shaped areas. Note thatmost of the nodes appear to be umbilicated and thatthey are surrounded by a relatively translucentarea. An area from which the nodules have beenremoved is present and demonstrates how thelesions may be ' picked off' without disturbing theunderlying amnion or the neighbouring nodules.Fresh specimen x 25.

Aetiology. Most of the reported instances ofamnion nodosum have been in pregnancies inwhich there has been very little or no amnioticfluid. Any abnormality that prevents the secretionof foetal urine may be associated with absence ofamniotic fluid. Renal agenesis is the lesion mostfrequently associated with amnion nodosum and,in fact, has been present in nearly every recordedinstance of amnion nodosum. Professor Jeffcoateand J. S. Scott (1959) consider, from an analysisof their 14 cases, that amnion nodosum is not adevelopmental error of genetic origin associatedspecifically with renal agenesis. Their conclusion

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200 POSTGRADUATE MEDICAL JOURNAL April I962

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FIG. 9.-Amnion Nodosum. A typicalnodule of amnion nodosum show-ing attempts at whorl formation.The lesion is not attached to theamnion, part of whose epitheliumpersists at the edges, beneath it.This particular node gives theimpression of having been de-posited upon the amnion as apre-formed nodule. The remainderof the amnion and the chorion(C) are normal in this instance,except for extensive depositionof fibrin in the trophoblast. T =Trophoblast. D = Decidua. B.V.= Maternal blood vessel. Haema-toxylin and Eosin x 75.

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FIG. io.-Necrosis of the cervical amnion in a patientwhose membranes ruptured spontaneously at 32weeks' gestation. The epithelium (E) is reducedto a continuous narrow bandet of eosinophilicamorphous tissue in which the remnants of threenuclei are visible. The remaining layers form arelatively continuous pale-staining strip ofreticular tissue containing no cells. Comparewith Fig. 5. Haematoxylin and Eosin x 390.

is supported by finding one instance of amnionnodosum in the presence of a normal renal tract,although the remaining 13 infants had renalagenesis. They consider that the lesion is actuallycaused by, or associated with, oligohydramniosrather than being the direct result of congenitalrenal abnormality.The exact mechanism whereby the nodules are

formed is uncertain. Squamous cells, normallyshed from the fietal skin, float freely within theamniotic fluid, unless or until, they are swallowedby the ftetus. When a shortage of liquor preventsfree movement of these cells they coalesce intotiny rounded coagula which become adherent to

the amnion producing secondary degenerativechanges in the amniotic epithelium.Premature Rupture of the MembranesThe aetiology of premature labour is in-

adequately understood and many investigationshave been undertaken in attempts to find the causeor causes. Unfortunately there has not beenagreement regarding the classification of theaetiological factors involved, with the result thatcorrelation of published findings is very difficult.Most authors stress the importance of prematurerupture of the faetal membranes as a causativefactor in the onset of premature labour. A search

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April 1962 BOURNE: The Fcetal Membranes 201

through the copious literature shows that pre-mature rupture of membranes is thought to be amajor aetiological factor in from 6% to 60% of allcases of prematurity although some authors do noteven include it amongst the possible causes.The author found that premature rupture of

membranes, as a major causative factor, occurredin 34% of a series of I,ooo premature infantsdelivered at the City of London MaternityHospital and the North Middlesex Hospital. Ina number of these cases there were both maternaland foetal factors which contributed to the onsetof labour, such as pre-eclampsia, multiple preg-nancy, anaemia, foetal abnormality and malnu-trition, etc. but, even so, in 34% the membranesruptured before the onset of true labour and musttherefore be considered to be a major factor inits onset.One of the main functions of the amnion and

chorion is to maintain the fluid environmentupon which the foetus depends for its survival inutero. There is little correlation between theirpremature rupture and other known factors,although the association between the youngprimigravida and the under privileged patient hasbeen noted; so also has its recurrence in sub-sequent pregnancies of a patient so afflicted inan earlier pregnancy. There is, however, still nodefinite link with any known clinical condition toaccount for the majority of instances.The incompetent cervix is now associated with

a syndrome that includes painless rupture ofthe membranes in the second trimester of preg-nancy. In some patients the membranes mayrupture when the cervix is only one finger dilated,whilst in others the bulging membranes may fillthe vagina before they finally break. Similarly,patients who abort during this period may rupturetheir membranes early, whilst others deliver thefoetus within a complete sac. These apparentinconsistencies are not entirely explained by theprotection which the well-placed presenting partis supposed to give to the foremembranes.

Similar conditions may be found at any sub-sequent stage of pregnancy. Why should the

membranes rupture at 32 weeks, or at term, in a

patient whose cervix is shut tightly and not yettaken up, whilst in another patient of similarmaturity they need to be artificially ruptured atfull dilatation of the cervix? It is this apparentincompatibility that has led many investigatorsto consider that such accidents are the result of aninherent weakness of the membranes themselves.

Examination of the tensile strength of the mem-branes has failed to demonstrate any significantdifference between the strength of those mem-branes that rupture prematurely and those thatdo not. It is impossible for these estimations oftensile strength to be undertaken upon theprecise area of rupture, partly because it cannot beexactly defined but also because it has alreadyruptured.A method has been devised (Bourne, 1962) bywhich the membranes presenting at the internal

cervical os can be stained in situ and then studiedhistologically after delivery. This means that theamnion and chorion can be studied at their exactsite of rupture. Using a satisfactory series ofcontrols it has been demonstrated that about 90%of the membranes that rupture prematurely,between the 3oth and 34th week of pregnancy,have previously undergone extensive degenerativechanges at their precise site of rupture over theinternal cervical os (Bourne, 1962).Such atrophic and degenerative changes in the

amnion and chorion (Fig. io) overlying theinternal cervical os would almost certainly changeboth the function and strength of the membranesin this area. It seems reasonable to suppose thatsuch changes would weaken their tensile strengthand hence lead to their premature rupture. Thereason for the necrosis of this particular area ofthe membranes is unknown and work is proceed-ing in an attempt to isolate the atiological factors.These findings await confirmation from other

workers, but they do, perhaps, provide a new andinteresting approach to the ever-increasing prob-lem of prematurity.

Figs. I, 2, 3, 4, 5, 6, 9 and io are reproduced from 'The HumanAmnion and Chonon' by kind permission of Lloyd-Luke (MedicalBooks) Ltd.

REFERENCESBOURNE, G. L. (1962): The Human Amnion and Chorion. London: Lloyd-Luke (Medical Books).(1960): The Microscopic Anatomy of the Human Amnion and Chorion, Amer. J. Obstet. Gynec., 79, 1070., and LACY, D. (1960): Ultrastructure of Human Amnion and Its Possible Relation to the Circulation of AmnioticFluid, Nature (Lond.), I86, 952.HOFBAUER, J. (1905): Grundziige einer Biologie der menschlichen Plazenta. Leipzig: W. Braumiiller.JEFFCOATE, T. N. A., and SCOTT, J. S. (1959): Polyhydramnios and Oligohydramnios, Canad. med. Ass. J., 8o, 77.KLINGER, H. P. (I957): Sex Chromatin in Fetal and Maternal Portions of the Human Placenta, Acta. anat. (Basel),30, 37I.LANDING, B. H. (1950): Amnion Nodosum: A Lesion of the Placenta apparently associated with Deficient Secretionof the Fetal Urine, Amer. J. Obstet. Gynec., 60, 1339.MOLLER, H. (1847): Abhandlung fiber den Bau der Molen. Wiirzburg: Bonitas-Bauer.PLENTL, A. A. (I957): In Gestation: Transactions of the Fourth Conference. Ed. C. A. Villee. New York: JosiahMacy Jr. Foundation, p. 71.

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