Paper - The Formation of the Connecting Stalk and the Extension of the Amniotic Cavity towards the Tissue of the Connecting Stalk in Young Human Embryos

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By J. FLORIAN, M.D. Masaryk University, Brno

From the Department ofAnatomy and Embryology, University College, London

V. Mollendorff first drew attention to the relation of the ectoderm to the mesoderm of the stalk in very young human embryos; in the description of the embryo OP (1921) he describes a "Zerfallende Epithelwucherung des Amnions" (1921, pp. 416, 417; fig. 9, p. 415) behind the caudal end of the embryonic plate, in which the epithelium degenerates and so the amniotic cavityisenlargedatitscost. In the course of the study of a number of young human embryos I have had the opportunity of convincing myself of the correctness of the above- mentioned discovery, and shall now try to give a description of my own observations. In the Fetzer embryo, the detailed description of which I shall shortly publish in collaboration with Dr Fetzer, as well as in the embryo Bi I (Florian, 1927) and T.F. (Florian, 1927, 1928 b) I was able to observe, in the neigh- bourhood of the tip of the amnion, granules in the mesoderm which can only beinterpretedastheresultofthedissolutionoftissueprovingthecorrectness ofv.M6llendorff'sobservation. IntheembryoBiI,Ishalldescribethe region in question in detail; in the present paper I shall occupy myself only with the description of the relations between the stalk ectoderm (I use this name in contradistinction to the amniotic ectoderm of which it is the direct continuation) and the stalk mesoderm in the embryo Bi II (with four paired somites)andBiXI (withtenpairedsomites).Thestalkectodermisseparated from the ectoderm of the embryonal disc by the cloacal membrane; it wil, therefore,alsobenecessarytotakeintoconsiderationthesurroundingsofthe cloacalmembrane inour description. Inorderthatthereadermay beabletofollowmy observationsmore easily, I shall begin with a short description of the early development of the human embryo and its connecting stalk. Stage 1 (text-fig. 1) corresponds with the Peters embryo (1899; Grosser, 1913): the primitive streak primordium and the stalk are not yet present; the embryo is situated in a thickening of the chorionic mesoderm. The future

Text-figs.1-9. Schemesofthechiefstagesinthedevelopmentofthehumanembryo(pp.454-61). Ectoderm black; entoderm lined horizontally, primitive streak vertically, head process and chord plate obliquely. Mesoderm dotted. The axis of the connecting stalk is marked by an arrow. The limits of the connecting stalk are marked by * and ®, the limits ofthe umbilical stalk by + and 0. The direction of the extension of the amniotic cavity towards the con- nectingstalkismarkedbyarrows.Thepartsoftheallantoisandoftheaxisoftheconnecting stalksituatedoutofthemedianplanearefinelydotted. A.Connectingstalk;B.umbilical stalk;Cl.m.cloacalmembrane.

limits of the stalk are designated by a ® and a *. The axis of that part of the chorionic mesoderm, which will be transformed into the stalk in the course of further development, is marked by an arrow: it is perpendicular to the middleoftheembryonalplate. During the further development, the amniotic cavity and the yolk-sac enlarge so rapidly that the mesoderm covering them is not able to keep pace. Whilst in stage 1 (text-fig. 1) the mesoderm of the amnion and that covering thedorsalmarginalpartoftheyolk-sacaresimilarinstructuretothechorionic mesoderm, in stage 2 (text-fig. 2) the mesoderm covering the whole yolk-sac and the cranial end of the amniotic cavity has taken on a different structure, andisbecomingtransformedintoa"mesothelium."Thecelsofthemesoderm in this region are connected with the cells of the amniotic ectoderm and with those of the yolk-sac entoderm by means of cytodesmata, and some of the cells of the yolk-sac entoderm appear to have entered the mesoderm. The transitionofthemesodermallayerwhichcoversthecranialendoftheamnion into the chorionic mesoderm (text-fig. 2*) represents the cranial limit of the connectingstalk,whichfirstbeginstobeformedinthisstage.Thefirsttraces of the stalk appear, therefore, in the stage when the primitive streak pri- mordium is represented by a localised fusion of the embryonal ectoderm with theentodermoftheyolk-sac(embryoOP v.Mollendorff).Theyolk-sac(in this stage) runs out towards the connecting stalk in the form of a slight diverticulum. Alreadyinthisstagev.Mbllendorffdescribed(1921,1925)an active extension of the amniotic cavity towards the stalk tissue (in a dorsal and caudal direction from the embryonal plate). If we regard the caudal limitoftheconnectingstalk(® intext-figs.1-8)asafixedpoint,wemaysay that the embryonal plate grows in the cranial direction, while the amniotic cavity expands caudally and dorsally. The yolk-sac correspondingly grows forwards below the embryonic plate almost up to the cranial end of the latter, andalsogrowsinthecaudaldirection. Itsgrowthinthisdirection,however, encounters the resistance of the connecting stalk tissue and so, not being able to extend freely, it grows towards the connecting stalk in the form of a diverticulum. Already in this stage we can distinguish two parts in the connecting stalk which I propose to term the amnio-embryonal stalk and the umbilical stalk respectively. The amnio-embryonal stalk (between ® and * in text-figs. 1-8) attachestheembryonalformation(amnio-embryonalandyolk-sacvesicles)as a whole to the chorionic mesoderm and continues on the one hand into the amniotic mesoderm, and on the other into the umbilical stalk. The umbilical stalk (between + and ® in text-figs. 2-8) represents a partial continuation of the amnio-embryonal stalk; on its cranial surface it is covered by the stalk ectoderm which is continuous with the ectoderm of the embryonic plate and with that of the amnion, and its whole surface can only be seen if the amnion is opened up. The most important difference between the amnio-embryonal stalk and the umbilical stalk lies in the fact that the latterisinalstagescoveredonitscranialsurfaceby ectodermand thatitis transformed into the umbilical cord during later stages of development. The amnio-embryonal stalk on the other hand persists for a time but finally disappears when the amniotic mesoderm fuses with that of the chorion. The axis of the connecting stalk (marked by an arrow) begins already in stage2(text-fig.2)toformanacuteangle(opencaudally)withtheembryonic plate and has moved slightly towards the caudal end of the embryo. 458 J.Florian Instage3(text-fig.3,correspondingapproximatelywiththeFetzerembryo (Fetzer-Florian, 1929)) both the amnio-embryonal and the umbilical stalks are quite distinct. The axis ofthe connecting stalk has now come to liebehind thecaudalendoftheembryonicplateanditisbeginningtoformacurvewith the convexity directed caudally. The amniotic cavity runs out into a distinct amniotictipcaudallyanddorsally,afeaturewhichistobeseeninallfollowing stages. The extension of the amniotic cavity towards the tissue of the con- necting stalk takes place in an opposite direction to that of the embryonic plate, the result being, that the distance between the amniotic tip on the one handandthechorionicectodermandthecaudallimitoftheamnio-embryonal stalk on the other increases at a much slower rate than the distance between the cranial end of the embryonic plate and the above-mentioned structures. In this stage, approximately in the middle of the embryonic plate, there is present a distinct primordium of the primitive streak and, close to the caudalendoftheplate,thecloacalmembraneisalreadydeveloped. Itis noteworthy that these two structures are in no way connected in their origin in the human embryo. The primitive streak primordium seems to correspond initspositionwithHensen'sknot. Inthisstage,theyolk-sacdoesnotsend any distinct diverticulum towards the stalk. In stage 4, represented by the embryo Bi I, of which a schema is shown in text-fig. 4, the bending of the axis of the stalk has become more distinct, and its tip forms a sharper angle with the embryonic plate than in the preceding stage. Intext-fig.4Ihaveshownanamnioticductconnectingtheamniotic ectoderm with the chorionic ectoderm, because two human embryos (BiI, Florian, 1927 and Strahl-Beneke, 1910) belonging to this stage possess it. It is not certain whether it is only a chance that the amniotic duct is not present in the embryos represented by text-figs. 1-3. The knot which re- presentedtheprimitivestreakprimordiumintheprecedingtwostageshasnow becometransformedintoatrue"streak,"attheendofwhichthereisadistinct "end-knot of the primitive streak." The thickened end-knot produces a ventral down-bulging of the underlying wall of the yolk-sac and so a diverti- culum is produced, at the caudal end of which the cloacal membrane is situated. This diverticulum resembles an allantoic outgrowth-with which, however, it has nothing to do, because it disappears during the later develop- ment by opening out into the yolk-sac cavity (Florian-Volker, 1929). Instage5(text-fig.5,correspondingapproximatelywiththeStieveembryo "Hugo," 1926) the tip of the bent axis of the stalk has passed stilfurther caudally.Theprimitivestreakoccupiesthewholecaudalhalfoftheembryonic plate and the primordium of the head process has appeared. The end-knot of the primitive streak has disappeared, and the above-mentioned caudal diverticulum has become merged in the yolk-sac cavity. In stage 6 (text-fig. 6, corresponding approximately with the embryo Peh.1-Hochstetter, Rossenbeck, 1923) a new diverticulum is seen extending almost vertically upwards from the caudo-dorsal extremity of the yolk-sac FormationofConnectingStalk,etc.,inHumanEmbryos 459 intothestalkmesoderm. Iproposetospeakofthisastheallanto-enteric diverticulum since its proximal part represents the later hind-gut, its distal portiontheentodermalallantoiccanal.Theproximalpartcoincidesinposition withthecloacalmembrane,furnishingitsentodermallayer,whilsttheallantoic canalissituatedcaudallytothatmembrane. The allanto-entericdiverticulum in this stage appears at first sight to be very similar to the diverticulum described in stage 4, which has been regarded by many authors, including myself, as the allantoic primordium, but there is a very important difference between the two, apart from the fact that the one is transitory, the other permanent. The cloacal membrane is situated in stage 4 at the end of the diverticulum there present, but in stage 6 it lies at the opening of the diver- ticulum into the yolk-sac cavity. The part of the diverticulum caudal to the cloacal membrane (the above-mentioned "allantoic" canal) is, therefore, in stage 4 not yet present. The axis of the stalk is now situated caudally to the amniotic cavity and its tip is directed to the upper or caudal end of the allantoiccanal. In stage 7 (text-fig. 7, corresponding approximately with the embryo Bi II) theaxisofthestalkliesstilmorecaudally,beingnow situatedbehindthe allanto-enteric diverticulum. The cloacal membrane is divided into two parts: one of them lies at the orifice of the diverticulum, the other is situated more caudally. In stage 8 (text-fig. 8, corresponding approximately with the Sternberg embryo (1927a),withfourpairedsomites)thecranialpartofthecloacalmem- brane is situated in the now very distinct primordium of the hind-gut. The other part lies close behind the orifice of the allanto-enteric diverticulum into the yolk-sac; it is now in process of degeneration and has become separated intothreepieces. Whereasinstage7,theallanto-entericdiverticulumlies wholly in the median plane of the embryo, in this stage the lengthening of the stalk has permitted a slight rotation of the embryo around its long axis, with the result that the end of the allantoic canal is no longer in the median plane. The mesoderm between the allantoic canal and the stalk ectoderm is now much thicker than in the preceding stage. The axis of the stalk now makes such a sharp curve that its tip runs nearly parallel to the long axis of theembryo. In stage 9 (text-fig. 9, embryo Bi XI with ten paired somites) certain important advances have been made in the region under discussion, (a) the caudal end of the embryo now projects freely backwards as the result of the developmental processes already initiated in stage 7, (b) the connecting stalk has increased greatly in length and, as the result of the growth of the tail-end oftheembryoandthedevelopmentofthehind-gut,hasbeencarriedventrally so that it now arises from the ventral side of the embryo, in front of the freely projecting caudal end of the latter, (c) as the result of the lengthening of the stalk,theembryohasturneditsrightsidetowardsthechorion,(d)theopening of the allantoic canal is now situated on the floor of the hind-gut. The tip of 460 J.Florian the axis of the connecting stalk is once more perpendicular to the axis of the embryonic body, but it has made a rotation of 1800 in comparison with the stage1. We cansee,therefore,thatthemigrationoftheinsertionoftheumbilical stalkfrom the dorsal-caudal to the ventral aspect of the body of the embryo is the resultpartlyoftheactivegrowthofthestalkandpartlyofthegrowthofthetail-end ofthe embryo in the caudal direction and the correlated extension ofthe amniotic cavityinthesamedirection. We seefurtherthattheembryoneednotchange its position during this process. The definitive cloacal membrane is now establishedinthefloorofthehind-gut. Itisformedbythecranialpartofthe cloacal membrane of stages 7 and 8. The whole allantoic diverticulum of this stage corresponds to that part of the allanto-enteric diverticulum in stage 7 whichissituatedbehindthedefinitivecloacalmembrane. Onlythepartof the allantoic diverticulum which is situated close to its opening into the hind-gut is situated in the median plane as the result of the rotation of the embryonic body round its long axis. If we compare the cloacal membrane in text-figs. 7 and 8, we notice that thedefinitivepartofthecloacalmembrane issituatedinstage7(text-fig.7)- and the same is true for stage 6 (text-fig. 6)-partly in the allanto-enteric diverticulum, but in stage 8 (text-fig. 8) the definitive cloacal membrane is now situated in the hind-gut. This fact justifies the conclusion that the part oftheallanto-entericdiverticulumclosetoitsopeningintotheyolk-sacreally belongsnottotheallantois,buttothehind-gut. InthisconnectionIcall attention to the characters of the entoderm in this region of the embryonic body (p. 467), which support my interpretation. If that interpretation is correct, it is clear that we cannot determine with certainty the cranial limit of the allantoic canal in stages prior to stage 8. This is one of the most remarkable differences between the development of the Man (and no doubt that ofalthe higher Primates) and the development oflower Mammals, and it is conditioned by the presence of the connecting stalk and the very early differentiation of the primordia of the cloacal membrane and the allantois. Only when the insertion of the umbilical stalk to the ventral part of the embryonic body (stage 9, text-fig. 9) is attained and the hind-gut is formed canthehuman embryo,withregardtothecloacalmembraneandtheallantois, be compared with that of a lower Mammal. Now I shall try to support my views by an account of my own ob- servations. Ishallbeginmy remarkswiththedescriptionofthecloacalmembranein the Embryo Bi II, in which I have previously described the chorda canal (1928 a). The embryo was cut cranio-caudally. The graphic reconstruction of the caudal end of this embryo (text-fig. 11) shows that the hind-gut is already forming,butisstilveryshallow.The cloacalmembrane isnotfoundinthis hind-gut primordium, but in that region where the yolk-sac passes into the allanto-enteric diverticulum, the greater part of which corresponds with the allantoisoflaterstages. CaudallytothetruecloacalmembraneIfounda FormationofConnectingStalk,etc.,inHumanEmbryos 461 connection between the amniotic ectoderm and the entoderm of the allanto- enteric diverticulum (between sections 315 and 314), the significance of which Ishalldiscusslater. HereIwishonlytocallattentiontotheregionbetween the cloacal membrane and this connection. In this region, the embryo Bi II shows nearly the same relations as the Sternberg embryo (1927 a), with which it agrees in the number of somites. Text-fig. 10. Embryo Peh.,-Hochsteter(Rossenbeck, 1923) according to Sternberg (1927b). x113. Ectodermblack;entodermlinedhorizontally,primitivestreakvertically. Mesoderm dotted. Sternberg has described in his specimen (text-fig. 12) behind the cloacal membrane (whichisfoundinthehind-gut)threeseparateconnectionsbetween the amniotic ectoderm and the entoderm of the allantois, and has regarded theseastheremainsoftheoriginalcloacalmembrane. Isuggestedin1929 (Florian and V8lker) the possibility that these connections are the result of secondary fusion of the amniotic ectoderm and the entoderm of the allantois, forthereasonthatthedistanceofthepointsoffusionfromtheactualcloacal membrane in the Sternberg embryo seemed to be much too great but, more recently, I believe that I have found a distinct transitional stage in the graphic reconstruction of the embryo Bi II (text-fig. 11) between the embryo 462t J.Florian , C, C OIr-t Wn _tIt s*P-1 M ID( C........ 4t. r OGto4 P- S -4-4mCO1 1 .. OD .. Ca

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C0~ P4 Ca FormationofConnectingStalk,etc.,inHumanEmbryos 463 Peh.1-Hochstetter (text-fig. 10) and the Sternberg embryo (text-fig. 12) and accordingly accept the view of Sternberg that they are actually remnants of the original much more extensive cloacal membrane. In the region between the actual cloacal membrane and the connection of theamnioticectodermandtheentodermoftheallantois,therelationsbetween the embryonal layers are very difficult to study, although the tissue is well preserved and the embryonal layers can be easily distinguished elsewhere. I, therefore, think it is necessary to describe this region somewhat more in detail. Text-fig.12.EmbryoSternberg(1927a,b). x113.Ectodermblack,entodermlinedhorizontally, primitivestreak,vertically,head-processandchorda-plate,obliquely. Mesodermdotted. In section 330 (Plate I, fig. 1) the cloacal membrane has its typical form; itissharplyseparatedfromthemesodermbyafairlybroadspace.Thevarious sized granules stained with haematoxylin, which are to be observed in great number in section 229 and which are possibly to be interpreted as a sign of celdestruction,areherepresentonlyinminimalquantities.We canaccordingly describe this section as the most caudal in which the cloacal membrane shows nonoticeablesignofdegeneration. Inthenext(inthecaudaldirection) section(No.229)thetissuecomposingthecloacalmembraneseemstobemuch lesscompact,especiallyclosetotheectoderm. (Itseemsthatwecanseehere thelimitbetweentheectodermaland entodermalpartofthecloacalmei brane,whichwasnotpossibleinsection330.) Intheentodermalpartof a cloacalmembranethereisacavityfiledwithvariouslysizedgranules;t e' I - .:" - , .- , .- - .-! - ~aQ -7 ~0) =:s CSC -Q -*-.v 04 cl. (I I 6 C. C.- L-q C C. -l-' n, -j C" .- :. r f". ,. -7 c., -1-1 .7 w\ -4 g JMark Hill (talk) 17:56, 3 July 2015 (AEST)

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\ V~~*Mark Hill (talk) 17:56, 3 July 2015 (AEST)0~0 \' J b ~- X .j;~0 FormationofConnectingStalk,etc.,inHumanEmbryos 465 granules contain enclosures stained with haematoxylin. They are possibly products of cell dissolution. On the rightthe cloacal membrane is separated fromthemesodermbyawidespace;on theleft,onthecontrary,themesoderin approaches much nearer to the cloacal membrane and is connected to it by finebutquitedistinctcytodesmata. Inthenextsection(328)theectoderm and the entoderm are connected by a single cell in the region of the cloacal membrane. Otherwisetheectodermisseparatedfromtheentodermbya distinct space, which is bridged over by fine cytodesmata. The granules referredtoasproductsofcelldissolutionareonlyvisibleintheentoderm. On the left, the mesoderin is quite near to the cloacal membrane, but one can easilydistinguishitfromthecloacalmembrane. Inthenextsection(327, Plate I,fig. 2) the ectoderm shows a fairly wide connection with the entoderm- wehave,therefore,beforeusadistinctcloacalnmembrane butifwecompare this section with the section 330, we can note certain differences. The dorsal surface of the cloacal membrane is not flat as in section 330, but shows a distinctpit,whilstitsleftborderismuch lessdistinctthantheright,because themesodermapproachesveryclosetothecloacalmembrane. Productsof celldissolutionarenottobeobservedinthecloacalmembrane, buttheyseem to be present in the mesoderm at a certain distance from it. In section 326 (PlateI,fig.3)therelationsoftheembryonallayersareveryindistinct. We can observe a cell cord which seems to connect the entoderm and the ectoderm, butthelimitbetweenthisandthemesodermiissoindistinctthatitcanscarcely be compared with the,well-developed cloacal membrane seen in section 330 (Plate I, fig. 1). Degeneration products are not visible in the cloacal mem- brane, but they are quite distinct in the mesoderm (on the left). In section 325 (Plate I, fig. 4) the limits between the cloacal membrane and the mesoderm are not much more distinct than in section 326, whilst degeneration products are very clearly visible in the ectoderni near to the amniotic cavity. In section 324 the limits of the cloacal membrane are not more distinct, and there are present in itvery many degeneration granules. In section 323 the ectodermn sends out towards the entoderm a narrow cell cord, between the end of which and the entoderm a degenerated cell is present. In section 322 this cell cord nearly reaches the entoderm; a very narrow space, possibly an artefact, lying betweenthetwo. Ontheleftofthiscellcord,theectodermisthickenedand thelimittowardsthemesodermisveryindistinct. Insection321(PlateI, fig. 5) there are only very indistinct traces of the above-mentioned cell cord. On the left the ectoderm is again thickened as in sections 324-2, and contains numerous granules. The limit between the ectoderm and the mesoderm in this sectionisnon-existent. Inthefollowingsections(inthecaudaldirection)there is no cell cord visible. The above-mentioned thickening of the ectodermi comes to occupy its position and assumes a triangular form; it contains many granulesanditslimittowardsthemesoderniisindistinct. Insection316the tip of this triangular thickening passes into a very distinct cell cord containing granules. Theselatterareespeciallydistinctinsection315(PlateI,fig.6) AnatomySHIV 30 466 J.Florian inanarrowcordwhichcanbetracedfromtheectodermtowardstheentoderm. Alreadyinthissection,butstilmore distinctlyinsection314 (PlateI,fig.7), the entoderm sends out a pointed process towards the ectoderm, which isvery likely connected with the ectodermic cell cord seen in section 315 and which, asindicatedabove,showsdistinctsignsofdegeneration. (Similarsignsof degeneration are also to be seen in degenerating amniotic ducts.) Inordertounderstandthedevelopmentofthehind-gut,itisofimportance to know the conditions at the cranial end of the cloacal membrane. The cloacal membrane shows no signs of degeneration in the cranial direction up to section332. Insection333acloacalmembraneisstilpresent,butinitscentre there is an oval cavity containing numerous granules, staining similarly to thoseinthecaudalregionofthemembrane. Insection334thelatterismuch narrower than in the preceding section. The hinder end of the amniotic cavity extends as a narrow horse-shoe shaped diverticulum ventral to the caudal end of the embryo, immediately in front of the cloacal membrane. The ectoderm in this section is distinct and is separated from the mesoderm, except over a very narrow region in the middle line, where these two layers are fused; we have to do here with the caudal end of the primitive streak, whichstreakisstiltobeseenalsoinsection335.The amnioticcavityisstil present in this section, but appears as two separated clefts, representing the continuation of the limbs of the horse-shoe shaped diverticulum in the preceding section. The ectoderm and entoderm are connected in the middle line by a cell mass, in which are present numerous granules, so that we have here the cranial extremity of the cloacal membrane, but the ectoderm is very indistinct, because it is fused dorsally with the mesoderm of the primitive streak and ventrally with the entodermn in the region of the cloacal membrane. In section 336 (Plate I, fig. 11) we find the same relations, but there are no granulesinthecloacalmembrane. Insection337theamnioticcavityisstil present as a single small, but sharply limited cleft. The ectoderm is quite indistinct. Insection338(PlateI,fig.12)neitherananiiiioticcavitynorthe ectodermn can be distinguished. The entoderm is here fused with what is certainlythemesodermoftheprimitivestreak. Inthemesoderm,closetothe entoderm, there are present granules of variable size. In section 339 the entodermn of the hind-gut is also fused with the primitive streak mesoderm. The observations recorded above justify us in drawing the following con- clusions: the cloacal niembrane shows very distinct signs of degeneration over aconsiderableextentofitscaudalregion. Productsofceldissolutionare also present in the cranial region of the cloacal membrane, but they are here much less abundant than in its caudal end. At its cranial end the cloacal membranepassesoverwithoutanylimitintotheprimitivestreak. Cranial tothecloacalmembranethemesodermisincontinuitywithboththeectoderm and the entoderm and a migration of cells from the entoderm into the meso- derm seems to be very probable in several places. In this connection we would call attention to the form of the entoderm FormationofConnectingStalk,etc.,inHumanEmbryos 467 celsintheneighbourhoodofthecloacalmembrane. The studyoftheentoderm in this region is not quite easy, because the entoderm is cut in most places obliquely; nevertheless we can conclude that the entoderm, cranial to the posterior intestinal portal(inthe mid-gut as well as ilthe yolk-sac), coIsists usuallyofcubicalcels,thoughinplacesthecellsareflattened. Intheposterior intestinal portal and in the hind-gut the entoderm cells assume a higher, even cylindrical form, and their cytoplasm stains deeply. The entoderm of the yolk-sac,elsewherelow,ishigherintheregionofthecloacalmembrane and wheretheyolk-sacpassesoverintotheallanto-entericdiverticulum. Inthe most caudal part of the hind-gut the entoderm is lower again, but it cannot be studied easily because of the obliquity of the sectional plane. The entoderm isespecially high in the caudal part of the cloacal membrane, wheretherearesignsofthedegenerationofthecloacalmembrane. Inthe ventral part of the orifice of the allanto-enteric diverticulum the entoderm containsnumerous granules. The above-mentioned allanto-enteric diverticulum of the yolk-sac has in transverse section a circular outline near its opening into the yolk-sac. Caudally to section 322 it is oval with its long axis runmiing dorso-ventrally. Starting from this section. the entodermal cells in the ventral wall of the diverticulum contain small basally situated vacuoles; the nuclei lie towards the free surfaces of the cells where the cytoplasm stains deeply like that of thecellsinthedorsalwallofthediverticulum. Caudallythecavityofthe allanto-enteric diverticulum assumes an elongated oval form (Plate I, fig. 7); thebasalvacuolesappearalsointhecellsofthedorsalwall. Inthiscaudal region of the allanto-enteric diverticulum, the cells become reduced in height. The form of the entoderin cells in the caudal region of the allanto-enteric diverticulum of embryo Bi II corresponds with that of the cells over the whole extentoftheallantoisinembryo BiXI (PlateI,figs.9,10). The significanceoftheseobservations Ihave discussed above (p. 460); now we may pass to the description of the cloacal membrane in embryo Bi XI. The embryo Bi(ttmann) XI, of whose caudal end a graphic reconstruction is shown in text-fig. 13, was obtained by an operation by Dr 0. Bittmann in Brno and was fixed intact in the chorionic vesicle in Zenker's solution im- mediately after the operation. About four hours later the chorionic vesicle was carefully opened. The specimen was imbedded in paraffin and cut nearly transversely in the cranio-caudal direction (the sections are 7,u thick). The entire embryo is well preserved and without any deformation; itpossesses tenpairedsomites. Iintendtodescribeitindetaillater,andnowIshalldirect attentiononlytothecloacalmembrane and therelationsbetweentheectoderm and the mesoderm of the stalk. The cranial end of the cloacal membrane is in embryo Bi XI indicated already in section 272, where there is no mesoderm between the ectoderm and the entoderm in the median plane and the ectoderm nearly reaches the ento- derm. In sections 273 and 274 a triangular process of the ectoderm meets 30-2 468 J. Florian theentodermandfusionofthetwolayershasprobablybeeneffected. Butthe cloacal membrane is quite definitely formed in section 276. There are no granules regarded as products of cell dissolution to be observed in sections 276 to 279, and no other signs of degeneration of the cloacal membrane are present. Here and there in the cranial end of the cloacal membrane the ectoderm is loosened from the entoderm, but such places are always very small, and they are also to be seen in mid-region of the membrane and accordingly are not toberegardedasindicationsofdegenerationofthemembrane. Insection280 celgranules firstappear, but they are always very few in number and much smaller than those in embryo Bi IL. The cloacal membrane can be followed up to section 294 without any difficulty(insection293thereisamitosisinoneofitscels). Insection295 the cloacal membrane is much more indistinct than in the preceding section, because the mesoderm on both sides comes close up to it and the area of fusion oftheectodermandentodermisgreatlyreduced. Inthefollowingsectionthere isa cellmass whichconnectstheectodermandentoderm, butitscelsseemto belong to the mesoderm; we have, therefore, reached the transition region where the cloacal membrane passes into the primitive streak. From the cloacal membrane up to the head-process the three embryonal layers are in continuity with each other in the median plane. It is not necessary here to enter into detailed comparison of the condition ofthecloacalmembraneinmy specimenswiththatdescribedinotherhuman embryos. IneedonlyreferthereadertothepaperbyFlorian-Vdlker,1929, in which the cloacal membrane in other very young human embryos is de- scribed, and the papers of Sternberg (1927 a, 1927 b). It would be superfluous to repeat the details here. Before I proceed to discuss the significance of my observations, I shall describe the conditions behind the cloacal membrane and, more particularly, the relations between the ectoderm and mesoderm of the connecting stalk in both my specimens, beginning with the embryo Bi II. In this embryo the interpretation of the sections is made difficult by the obliquity of the sectional plane. The result of this is that the limit between the ectoderm and mesoderm isinsomeplacesobliterated. Butitseemsveryprobablethattheindistinct limit of the mentioned layers is not simply the result of the mentioned obliquity, but that there is actually no distinct limit between them. The ecto- derm of the stalk consists of a layer of flattened or low cubical cels, which exhibits only rarely a distinct limit towards the mesoderm and, even in these places, the ectodermal cells are connected with the mesodermal by means of numerous fine cytodesmata. We may divide the stalk into two regions: a ventral one extending to the level of the caudal end of the allantois, and a dorsal one found dorsally to the latter. I shall describe the ventral region of the stalk first. In many places the stalk epithelium consists here of three or more cell layers, of which the cells of the basal layer are connected with the underlying mesodermal cells by FormationofConnectingStalk,etc.,inHumanEmbryos 469 meansofprocesses. Itisimpossibletoprovetowhichgermlayerthesingle celsofthisepitheliumbelong. Onepossibilityisthatwehaveheretodowith anectodermconsistingofmany celllayers,andthatthecellsofthebasallayer are connected with the mesodermal cells; in several places the axes of these basalcellsaresodirectedastosuggestthepassageofectodermiccellsintothe mesoderm. Butthereisalsoanotherpossibility:thecellsofthebasallayer may belong to the mesoderm, although they are so closely related to the ectoderm. Itisespeciallydifficulttofindthelimitbetweentheectodermand mesoderm close to the median plane in the region between the ectoderm and the allanto-enteric divertieulum, as was mentioned in the description of the cloacalmembrane. We canseethisregion(closetothemedianplane)on the rightoffig.8(PlateI). On theleftofthesamefigurethereisacelltobeseen, which seems to belong to the stalk ectoderm and which is connected by means of a xvery long and distinct process with the mesodermal cells (Plate I, fig. 8 +). As we have mentioned above, the indistinct limit between the ectoderm and the mesoderm in this region is not only a result of the oblique plane of sections; it is very possible that some of the ectodermal cells pass into the mesoderm. Also in the dorsal part of the connecting stalk (dorsal to the level of the caudal end of the allanto-enteric divertieulum) the limit between the ectoderm and the mesoderm is very indistinct. The amniotic ectoderm close to the insertion of the amnion into the connecting stalk is distinguishable by its characters; whilst the amniotic ectoderm far from that insertion consists of asinglelayerofveryflattenedcels,theamnioticectodermclosetoitstransition intothestalkectodermisformedbyseverallayers. Itsbasallayerrepresents the direct continuation of the amniotic ectoderm of that part of the amnion remote from the insertion on the connecting stalk, and consists of a single, continuous layer of cels which has stained very deeply with cosin. The cells are cubical and their limits are very distinct. On this basal layer are situated polygonal or round cells whose cytoplasm has stained only faintly with haematoxylin. Intheircytoplasmtherearedistinctvacuolesespeciallyclose to the cel surface next the amniotic cavity. These cells are rather large and contain a large, round nucleus, which is larger and rounder than the nuclei of the basal layer. These cells are vcry often separated from each other by distinct spaces, and accordingly they do not form a continuous layer. The connection of the cells of this layer with the cells of the basal layer is in some places only very loose, and there is no doubt that they can be shed into the amniotic cavity where they undergo dissolution. The products of this cell dissolution can very easily be found in the amniotic cavity. The two above-mentioned cel layers of the amniotic ectoderm can be followed into the stalk ectoderm. The upper layer of the stalk ectoderm shows no particular differences as compared with the amnion in its neighbourhood, but the cells of the basal layer (deeply stained with cosin) enter into very complicated relations with the surrounding mesoderm. The form of cells of 470 J.Florian the basal layer is very irregular: some of them are cubical like the cells in the basal layer of the amnion, and there are many transitions between them andtheupperlayerofthestalkectoderm;itseemstobeverylikelythatthese cells will be shed into the amniotic cavity during the further development, and there dissolved. Theothercelsinthebasallayerretaintheireosinophilcharacter. Close to the insertion of the amnion into the connecting stalk, these cells send out long and broad processes-into the mesoderm and, as the result, they present peculiarformsandtheirnucleibecomeoval(PlateII,fig.14). Inthetissue closetotheectodermthereareinsomeplacesverydistinctgranules(PlateII, fig. 15). These seem to be a certain proof of the extension of the amniotic cavity at the expense of the stalk mesoderm; the ectoderm grows here deeply intothemesodermandcompressesitorbringsaboutitsdissolution. Inthis way theamnioticcavityisenlargedtowardsthetissueoftheconnectingstalk, this enlargement being specially marked and proceeding most rapidly close to the insertion of the amnion into the connecting stalk.,The convex outline of the surface of the connecting stalk directed towards the amniotic cavity (in the sections) isthe resultofthisprocess (Plate I,figs.13 and 16). Even in the region remote from the insertion of the amnion into the con- necting stalk there are present, underneath the stalk ectoderm, products of cell dissolution, so that even here the mesoderm undergoes an involution. The form of the ectoderm also differs very much in this same region from that close to the insertion (Plate III, fig. 17). The ectodermal cels are in places very flattened (like an endothelium) and vaulted towards the amniotic cavity so that the axis ofthe celforms here a curve convex towards the amniotic cavity. Eveninthisregionthereisevidencethatsomeoftheectodermalcells are eliminated into the amniotic cavity and dissolved. Such cells contain, even before they are eliminated, large vacuoles and their cytoplasm stains onlywithhaematoxylin. Itisofsomeinterestthatalsothemesodermalcells close underneath the ectoderm contain vacuoles. Inthissame regionthereareotherplaceswhereitisimpossibletoprove the existence of an ectodermal layer, the mesoderm alone appearing to bound theamnioticcavity. SuchplacesareintheembryoBiIIneververylarge. Inotherplacestheectodermisverydistinct;itscelsareverydeeplystained with eosin in contrast to the mesodermal cels, but the limit between the ectoderm and themesoderm isnot distinct,sincethecelsoftheformerare connected with those of the latter by means of cytodesmata. (Insection332Ihappenedtofindacelinthemesodermwhichwas much larger than the surrounding mesodermal cels, and deeply stained with eosin. Itseemsverylikelythatwe haveheretodowithanectodermalcelwhich hasemigratedintothemesoderm.) We may now passtothedescriptionoftherelationsbetweentheectoderm andthemesodermoftheconnectingstalkinembryoBiXI (text-figs.9,13). The flattenedamnioticectoderminembryoBiXI passesintotheectoderm FormationofConnectingStalk,etc.,inHumanEmbryos 471 oftheconnectingstalk,whichconsistsofmuch highercells;thesecellscannot be shortly designated as cubical or cylindrical, because their form is too irregular and we must, therefore, describe them more in detail. As in the embryo Bi II, we can, even in the embryo Bi XI, find in the stalk ectoderm cells which are eliminated into the amniotic cavity and dissolved. They appear to lie on a single layer of flattened cells (Plate III, figs. 22 and 23). These flattened cells in some places become higher and assume an irregular form. The line formed by the basal surface of these cells is not rectilinear as in a cubical or cylindrical epithelium, but quite uneven. The reason for this is to be found in the migration, in many places, of some of the ectodermal cells into the mesoderm (through the basal line of the ectoderm). They are to be found in varying stages of passage into the mesoderm (Plate III, fig. 22). We canveryoftenobserveecellswithhalfoftheirbodiesbelowthebasallineof the ectoderm and connected with the mesodermal cells by means of wide processes. In one place (Plate III, fig. 21), where the ectoderm forms a com- paratively regular layer and where the basal line isrectilinear, we find a cel, with one half of its body situated above the basal line, the other half below it, thetwohalvesbeingconnectedbyaverynarrowbridge. Eventhenucleusis divided into two halves connected by a very narrow, but quite distinct, bridge. Juston therightofthiscellthereisanothercellwhichsendsabroadprocess into the mesoderm, the sharpened end of its nucleus lying below the basal line. In other places the ectodermal cells form litle groups, penetrating into themesoderm. We canseeinPlateIII,fig.18suchagroupintheformof a triangle, the base of which lies at the upper surface of the ectoderm, whilst thetipisdirectedtowardsthemesoderm. Insuchagroupthecellsundergo a dissolution in such a way that finally only those cells remain which cover themesoderm; theinnercellsdisampearand intheirl)lacetherearisesasmall cavity connected with the anmniotic cavity (Plate III, fig. 19). This isone way in which the amniotic cavity is enlarged towards the connecting stalk. In order to elucidate further the mode of extension of the amniotic cavity towards the stalk mesoderm, I must describe a very interesting observation in embryo Bi XI. As is to be seen in Plate III, figs. 20 and 24, the right side of the embryo (in the younger embryos it is the dorsal side) is turned towards the chorionic membrane and the insertion of the amnion into the connecting stalk is ventral to, and on the right (in the figure) of, the caudal end of the embryo. The amniotic vesicle runs out towards the mesoderm of the connecting stalk in theform ofa wide bay, inwhich a freecellmass issituated. This celmass, according to its structure, belongs without any doubt to the mesoderm, but itiscompletelyseparatedfromthestalkmesoderm andliesfreeintheamniotic fluid. As is to be seen especially in Plate III, fig. 24, the mesodermal cells are comparatively well preserved in the middle of the mass, in contrast to its surface, where no cells arc visible. We have here, without any doubt, to do with a mass of mesoderm in course of dissolution, from the surface inwards. 472 J.Florian That this is so is indicated by the presence of numerous granules on the whole surface of the mass, and especially in that part of the surface which is turned towardsthestalkectoderm;here butonlyhere veryslighttracesofthe ectoderm are present on the surface of the mass which, in some places, touches the stalk cetoderm. In my opinion we can explain the presence of such a quite separated mesodermal mass in the amniotic cavity and its dissolution only in the following way. The stalk ectoderm penetrates into the mesoderm as above described and brings about its dissolution, where it is in direct contact with it, but it also can grow round a comparatively large mass of mesoderm and separate it completely from its surroundings. Such a sequestered mass remains for a time in the amniotic fluid, until it is completely dissolved. This is another and very intensive way, in which the amniotic cavity increases in extent at the expense of the connecting stalk. The observations recorded abovejustifytheopinionthattheextensionofthe amnioticcavityfirstmentionedbyv.M6llendorffintheveryyoung embryos OP and XVO takes place even in the embryo Bi XI with ten paired somites and that itiseveninthismuchmoredevelopedspecimeninactiveprogress. Aknow- ledge of the above-mentioned processes is very important for the understand- ing of the development of the connecting stalk in Mlan, as I have attempted to describe by means of the schematic median sections (text-figs. 1-9). In this connection I must make some mention of the literature bearing on this question. I have already had the opportunity (Florian, 1928 b, pp. 537-8) of de- scribingmy observationsonthestalkectodermofsomeyounghumanembryos. I was then inclined to accept the opinion of Fahrenholz (1927) that the amniotic cavity is not enlarged towards the tissue of the connecting stalk, but that, on the contrary, it grows out of the connecting stalk. But the sequestration of a part of the stalk mesoderm, described in embryo Bi XI, cannot be explained by Fahrenholz's view and, therefore, I am obliged to mention the Fahrenholz's observations more in detail. Here I cite the most important part of Fahrenholz's account (1927): "Auch bei dem Ei Ho sind die Degenerationserscheinungen im Amnionzipfel nachweisbar....Es listsichjedochindiesemFallzeigen,dassdieseVorgange nicht zu einer Erweiterung, sondern umgekehrt zu einer Verkleinerung der Amnionhdhle ffihren. Das Amnionepithel whchst nicht in den IHaftstiel hinein, sondern es zieht sich daraus zurick" (pp. 301-2). "Wir haben hier also einen Ilohlraum im Bindegewebe vor uns, der in der Nachbarschaft des Amnionepithels noch gut erhalten, an seinem distalen Ende aber bereits im Schwinden begriffen ist und noch Reste des Amnionepithels enthilt, das ihn friiher auskleidete. Der Spalt ist fiber neun Schnitte von 7 5,u Dicke zu verfolgen und reicht stellenweise bis an die Chorionmembran heran.....Ich muss derMeinungvonGrosser(1924)undBryce(1924)beipflichten,dass Amniongang usw. Reste eines primaren Zusammenhanges zwischen Amnion- FormationofConnectingStalk,etc.,inHumanEmbryos 473 und Chorionepithel sind. Diese Annahme allein bringt die Befunde unter einen Hut und gibt eine Erkldrung nicht nur fuirden Amnionzipfel, den Amniongang und die Zelldegenerationen, sondern auch fur den Amnionnabel und die Choriongange des Ilaftstieles, denen gegenuiber die v. Mollendorffs Theorieversagt. EineigentlicherAmniiongangfehltbeimEiHo. DieZellen des oben (S. 268) erwhihnten Epithelstranges im IHaftstiel (Fig. 30 und 31) unterscheiden sich durch ihren hellen, scharf umgrenzten Plasmaleib deutlich vondendunkelgefdirbtenZellendesAmnionepithels. Siegleichendagegen vollkommen den Elementen der Langhansschen Zcllschicht. Ich halte den Strang daher fireinen Chorionstrang" (p. 303). Thedifferencebetweenv.MCllendorff'sobservationsonembryosOP (1921) andWO (1925)ontheonehand,andFahrenholz'sobservationsontheother, could possibly be put in this way, that v. AMullendorff stu(lied the stalk ectoderm in that region, where an extension of the amniotic cavity towards the connecting stalk takes place, whilst Fahrcnholz devoted particular atten- tion to the place of the insertion of the amniotic duct into the stalk ectoderm. The fact that the epithelial cord, described in embryo Ho by Fahrenholz, consists of Langhan's cels, is not in contradiction with the opinion that we haveheretodowiththeremainsofanamnioticduct. Ihadtheopportunity to convince myself in embryo Bi I, where there is a continuous connection between the ectoderm of the chorion and that of the amnion, that the caudal part of the amniotic duct consists of Langhan's cells. Stump (1929) made the same observation in an embryo, in which there is the same continuous con- nection between the ectoderm of the chorion and the amnion, but which is much older than the embryo Bi I; it is the oldest embryo in which such a continuous connection has been described. I believe, therefore, that the "chorionic duct" described by Fahrenholz in embryo I-lo represents remains of an amniotic duct, and that Fahrenholz was dealing with the junction of the amnioticductwiththestalkectoderm. v. Mollendorff's opinion possibly does not explain the amniotic duct so well as Fahrenholz's view, but there are, I believe, other and more important difficulties raised by the view of Fahrenholz: it is not able, for example, to explain our observations on the stalk ectoderm of the embryo Bi ll and Bi XI. XVe ought to count also on the possibility of a combination of an involution of the amniotic cavity in the neighbourhood of the insertion of the amniotic duct and of an extension of the same in other parts of the stalk ectoderm. I may cite here another important observation made by Rossenbeck in the embryo Peh.1-Hochsteter (1923, p. 353): "Auffallend ist, dass funf Schnitte vor dem caudalen Ende der Amnionhohle ihre dorsale Wand wie zersprengterscheint,sodassman denEindruckhat,alswiirdesichierdirekt von dem Mesoblastgewebe des Bauchstieles begrenzt (vgl.Taf. XLI, Abb. 46)." I have already described places (p. 460) in the embryo Bi I, where there is also no ectoderm visible, but this region in the embryo Peh.1-IHochstetter seems to be much larger. In this connection I call attention to the outline 474 J.Florian of the connecting stalk towards the amniotic cavity in the sections of embryo Peh.1-I1-ochsteter; it is convex towards the amniotic cavity as it is also in the embryo Bi II. XVe could easily imagine that the stalk ectoderm could grow round and isolate a part of the stalk mesoderm with resulting sequestra- tion of the latter during the further development. If we compare the early development of the Man with the development of the other Mammals on the basis of the observations and interpretations set forth in the preceding pages, we reach the following conclusions: The most remarkable difference-the reason of al later-mentioned differences-between the development of Man and that of other Mammals, is to be seen in the very early development in the former of the mesoderm of the chorion and the very early differentiation of the connecting stalk and the structures related to it, viz. the primitive streak primordium, the cloacal membraneandtheallanto-entericdiverticulum. Further,thepresenceofthe ectoderm on the cranial surface of the umbilical stalk must be regarded as a preparation for the development of the umbilical cord. The existence of the stalk offers a certain resistance in the development of the caudal parts of the embryonic body and complicates that development in the following respects: (1) The extension of the amniotic cavity towards the tissue of the stalk (in the caudal and dorsal direction) can be effected only by means of an activity of the stalk ectoderm by which the mesoderm of the stalkispartiallydestroyed. (2)Theyolk-sacpenetratesintotheconnecting stalk in the form of a narrow diverticulum which enlarges and eventually opens out again into the cavity of the yolk-sac. This process may probably berepeatedseveraltimes. (3)Theveryearlyprimordiumoftheallantois does not arise directly from the hind-gut (as in the lower Mammals) but from an allanto-enteric diverticulum. The orifice of this diverticulutm later comes toformapartofthehind-gut. Itisonlyinthestagewhentheinsertionof the umbilical stalk has reached the ventral wall of the embryonic body that the allantois can be said to arise directly from the hind-gut, and so in this way the conditions characteristic of the lower Mammals are realised. I desire to express my grateful thanks to Prof. J. P. Hill for his advice and criticism and for reading the manuscript. The photo-micrographs have been prepared in the Department of Anatomy and Embryology and I desire to thank Mr F. Pittock for his great help. FormationofConnectingStalk,etc.,inHumanEmbryos 475 DESCRIPTION OF PLATES I-III Figs. 1-24. The figures are orientated so that the dorsal side is uppermost, unless otherwise stated. Al. allanto-enteric diverticulum; Amn. amniotic cavity; Amn'. diverticulum of the amniotic cavity below caudal end of embryo in the median plane; Cl. cloacal membrane; Bet.ectoderm;Emb.caudalendoftheembryo;Ent.entoderm;Ent'.entodermofthehind-gut; Ent". entoderm close cranial to the orifice of the allanto-enteric diverticulum; Ch. chorion; Ch.c. chorionic cavity (extra-embryonal coelom); Mes. mesodermal mass lying free in the amnioticcavity. PLATE I Fig.1.EmbryoBiII,section330. x300.Cloacalmembraneinitsbestdevelopedpart.Dorsal sideontheright,ventralsideontheleft. Fig.2.EmbryoBiII,section327. x400.Thelimitsofthecloacalmembranetowardsthe mesoderm are becoming indistinct. The limits of the ectoderm towards the mesoderm are indistinct laterally to the cloacal membrane. Text on p. 465. Fig.3.EmbryoBiII,section326. x400.Theprocessgivenoffbytheectodermtowardsthe entoderm of the allanto-enteric diverticulum is very indistinctly marked off from the meso- derm. Text on p. 465. Fig.4.EmbryoBiI,section325. x400.Thelimitsoftheremnantofthecloacalmembrane are very indistinct. Underneath + are distinct granules (sign of cell-dissolution). Text on p.465. Fig.5.EmbryoBiII,section321. x400.Theectodermisconnectedwiththeentodermbya singlespindle-shapedcel. Intheplacemarkedbyanarrowtheectodermisthickenedin a knot. Text on p. 465. Fig.6.EmbryoBiII,section315. x400.Theectodermgivesoffacell-cordwhichnearlyreaches theentoderm. Betweenitsendandtheentodermtherearenumerousgranules(possibly products of cell-dissolution); such are also visible in the ectoderm and entoderm. The dorsal Bide of this section on the right, the ventral on the left. Text on pp. 465, 466. Fig.7.EmbryoBiII,section314. x400.Theallanto-entericdiverticulumhasaformofa flattenedcanal. Inthedorsaldirection(totheright)itsendsashortprocesstowardsthe ectoderm,inwhichgranulesaretobeseen. Intheventralwalloftheallantois(ontheleft) theentodermalcellscontainvacuolesintheirbasalpart;thenucleiaresituatedtowardsthe freesurfaceofthecels.Textonpp.466,467. Fig.8.EmbryoBiI,section320. x520.Thelimitbetweentheectodermandthemesodermis very indistinct, especially in the thickened knot (*) close to the median plane. Below + there isanectodermalcelconnectedwiththemesodermbyadistinct,longprocess.Textonp.469. PLATE II Fig.9.EmbryoBiXI,section261. x400.Theallantoisclosebeneathitsorificeintothehind-gut. Entoderm oftheallantoisconsistsofvacuolatedcels. Dorsalsideontheright,ventralon the left. Text on p. 467. Fig.10.EmbryoBiXI,section276. x400.Allantoisfarcaudaltoitsorificeintothehind-gut. Otherwiseasinfig.9. Fig.11.EmbryoBiII,section336. x161.Textonp.466. Fig.12.EmbryoBiII,section338. x361.Textonp.466. Fig.13.EmbryoBiII,section326. x1661.Thepenetrationoftheectodermintothemesoderm (especially distinct at +). Text on p. 470. Fig.14.EmbryoBiII,section326. x520.Thedetailoftheplacemarkedby+infig.13. Text on p. 470. Fig.15.EmbryoBiIT,section322. x520.Numerousgranulesinthemesodermunderneath the ectoderm (close to the insertion of the amnion to the connecting stalk). Fig.16.EmbryoBiII,section335. x161.Textonp.470. 476 J.Florian PLATE III Fig.17.EmbryoBiII,section335. x480.Detailfromfig.16(theplacebetweenthetwoarrows in fig. 16). The ectoderm is apparently absent in some places. Fig.18.EmbryoBiXI,section301. x6131.Thepenetrationofthestalkectodermintothe mesoderm in the form of a thickened knot. Text on p. 471. Fig.19.EmbryoBiXI,section295. x613k.Aplacesimilartothatinfig.18.Textonp.471. Fig.20.EmbryoBiXI,section304. x52.Viewoftheamnioticcavity(Amn.)withthecaudal end of the embryo, the chorionic membrane (Ch.) and the connecting stalk (on the right). In an outpocketing of the amniotic cavity there is a sequestered mesodermic mass (Mes.). Fig.21.EmbryoBiXI,section284. x613k.Ectodermalcellspenetratingintothemesoderm oftheconnectingstalk(+,*). Textonp.471. Fig.22.EmbryoBiXI,section300. x6131.Flattenedectodermalcellspenetratingintothe mesoderm of the connecting stalk (for example at +). Some of them have been shed into the amniotic cavity (marked by an arrow). Fig.23.EmbryoBiXI,section298. x613k.Theflattenedcellsofthestalk-ectodermarecon- nected with the mesodermal cells by means of cytodesmata (+, *). Text on p. 471. Fig.24.EmbryoBiXI,section315. x93k.Thesequesteredmesodermalmass(Mes.)inan outpocketing of the amniotic cavity. This figure is turned through 900 in comparison with fig.20. REFERENCES FAHRENHOLZ, C.(1927). "EinjungesmenschlichesAbortiv-Ei." Zeitschr.f.mikr.-anat.Forschung, vol.vy. FETZER (1910). "Tber ein durch Operation gewonnenes menschliches Ei." Anat. Anz. vol. xxxvii,Erganz.-Heft. (Verh.d.anat.Ges.,Bruissel.) FETZER,M.andFLORIAN,J.(1929). 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(1925)."DasmenschlicheEiWO(lfring).Implantation,VerschlussderImplantationsoffnung und Keimesentwicklung beim Menschen vor Bildung des Primitivstreifens." Zeitschr.f. Anat.u.Entwickl.vol.LxxVI. PETERS,H.(1899). tberdieEinbettungdesmenschlichenEiesundda8frihestefisherbekannte menschlichePlazentationsstadium. LeipzigandWien. ROSSENBECK, H. (1923). "Ein junges menschliches Ei. Ovum humanum Peh.,-Hochsteter." Zeitschr.f.Anat.u.Entwickl.vol.LXVm. STERNBERG, H. (1927a). "Beschreibung eines menschlichen Embryos mit vierUrsegmentpaaren, etc." Zeitschr.f.Anat.u.Entwickl.vol.Lxxxia. - (1927b)."ZurformalenGenesederBauchblasenspalte(Exstrophiavesicae)." Virchows Archiv,vol.ccLxmi. STIEVE, H. (1926). "Ein 13k Tage altes, in der GeebArmutter erhaltenes und durch Eingriff gewonnenesmenschlichesEi." Zeitschr.f.mikr.-anat.Forsch.vol.Vi. STRAHL, H. and BENEKE, R. (1910). Ein junger menschlicher Embryo. Wiesbaden. STUMP,C.WITHERINGTON(1929). 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Reference

J Florian The Formation of the Connecting Stalk and the Extension of the Amniotic Cavity towards the Tissue of the Connecting Stalk in Young Human Embryos. J. Anat.: 1930, 64(Pt 4);454-476.5 PMID 17104291 | PMC1250149

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