Paper - The Early Development of Man, with Special Reference to the Development of the Mesoderm and Cloacal Membrane
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BY J, FLORIAN, M.D.
From the Department of Anatomy and Embryology, University Colege, London, and the Department of Histology and Embryology, Masaryk University, Brno
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The object of this paper isto give a short review of our present knowledge of the early stages in the development of Man, especially of the development of themesodermandthecloacalmembrane. Althoughaconsiderablenumberof earlyhuman embryoshavebeendescribedinthelastfortyyears,therearestil some important stages unknown and many important questions remain un- solved.Thereis,forinstance,thequestionofthepreciseageoftheknownhuman embryos of early stages. Valuable contributions on this subject have been made by Grosser,andthereaderwillfindaltheavailabledetailsinGrosser's papers of 1924 and 1927, and more recently v. Hayek (1931) and Grosser's pupil Treutler (1931) have made contributions to the solution of this difficult problem. The data thus far known, however, are insufficient to settle finally the question, in which period of the cyclic changes of the uterine mucosa the implantationofthehuman ovum occurs,andthelengthoftheintervalbetween ovulation and implantation, i.e. the duration of the migration of the human ovumfromtheovarytotheimplantationplace. Moreoverwehavenoaccurate knowledge of the details of the implantation process itself (see the different ideas described by Grosser (1924, 1927), Bryce (1924), Teacher (1925) and v. Mollendorff (1925). The lack of adequately preserved material makes it im- possibletoacceptfullyanyoneofthesedifferentexplanationsoftheimplanta- tionofthehuman egg.
The earliest stages of the development of the human embryo so far known are already too advanced to give us an idea of the structure of the human morula and early blastocyst and of the origin of the primary mesoderm. There areseveralspecimensdescribedwhichmustbeconsideredastheearliestknown human embryos. The following ova (arranged chronologically according to their description) come into consideration. Teacher-Bryce I (Bryce-Teacher, 1908; also Bryce, 1924), Kleinhans (Grosser, 1911 and 1922), SCH (v. Mollen- dorf, 1921a), Miller (Streeter, 1927) and the specimen described by Siegfried Mullerin1930. Itiscertainthatalthesespecimensaremuchlessdeveloped thantheembryoPeters(Peters,1899andGrosser,1913)ortheembryoWerner (Stieve,1931),butitisverydifficulttodeterminetheirexactsequence.Three of them are veryincomplete:fromtheseriesofsectionsthroughtheKleinhans ovum (Grosser, 1922) only a single section was preserved, containing only the trophoblast and the cavity of the blastocyst cut tangentially. The other sections,includingtheentireembryo,werelost.OftheMillerembryothereare preserved only five sections (only four of them contain the primordia of the two vesicles of the embryo), the sequence of which was not known though Streeterhasbeenabletoarrangethemintheirapparentserialorder. Ofthe S.Mullerspecimenonlythreesectionsarepreserved,oneofwhichissuggested by the author to represent the median section.
Of the first two specimens, one (Teacher-Bryce I) was obtained by abor- tion and was immersed in a mixture of blood and urine 20 hours before it wasfixedinalcohol. Althoughthepreservationofthespecimenismuchbetter than one would expect from its unfavourable history, some artificial deforma- tionoftheembryohasevidentlyoccurred.Thethirdofthefiveearliestembryos -ovum SCH v. Mdllendorff-shows such surprising conditions, so different from those one would expect on the basis ofother known embryos that ithas called forth doubts whether it is a normal ovum (see Grosser, 1924). The probability of artificial deformation is increased by the fact that the specimen is an abortive ovum.
Under such circumstances, itisreally impossible to determine the develop- mental sequence of these five specimens: the Kleinhans ovum can hardly be discussed because of the entire absence of the embryo. The embryo SCH is somewhatdoubtfulbecauseofitshistoryandpeculiarform. Ontheother hand, we must bear in mind that v. Mollendorff (1925) does not admit any deformation of the specimen on the ground of his own study of the sections. If he is right in this point, he is certainly justified in maintaining that the peculiar form of the embryo must not be used as a proof of the abnormality or deformation of the specimen.
The S.Mullerspecimenrepresentsperhapstheyoungestknown stageofthe development of the human embryo, but the loss of most of the sections con- siderablydiminishesitsvalue. Alsoitspreservationisnotperfect:thesupposed primary mesoderm of the chorion is detached from the chorionic ectoderm and an extensive artificial cleft is present between them; the continuity of the chorion, adjacent to the later decidua capsularis, is interrupted by a large opening and the shrunken and detached inner contents of the blastocyst are situated in this opening. The most probable explanation of these conditions is an artificial breach of the chorionic wall and the dislocation of the inner contentsoftheblastocystintothisartificialopening. Suchdeformationcan be easily explained as the result of pressure due to the mechanical damage during the treatment of the specimen or to the influence of the fixative. This explanation is supported by the great shrinkage exhibited by the specimen. On theotherhand,we must callattentiontothefact that if we acceptv.Mol- lendorff's hypothesis concerning the implantation of the human egg (see fig.18,p.403,1921aandfig.21a-d,p.36,1925),suchconditionsaswe can observe in the Muller specimen (the primordium of the mesoderm plus embryo is intercalated into the trophoblast which does not cover the former on its outer surface) would be quite normal. But, quite apart from the state of preservation of this specimen, we are not prepared to accept v. M6llendorff's hypothesis without hesitation.
The specimens Teacher-Bryce I and Miller represent nearly the same developmentalstage. v.Mollendorffsuggested(1921a,pp.369-70,359,384) that the Miller ovum might belong to a much more advanced developmental stage than was supposed by Streeter, and that the section published by Miller (1913)reallyisatangentialone. v.Mdllendorffcallsattentiontothepossibility of the existence of the connecting stalk in this specimen and thinks that the embryo in the Miller ovum isperhaps more advanced in itsdevelopment than that of the ovum Teacher-Bryce I, but it is very difficult to determine what was the actual form of the latter in the living specimen because of the un- favourable conditions of preservation, or the actual size and structure of the embryooftheMillerovumbecauseoftheincompleteseriesofsections. Inmy opinion, the trophoblast of the Miller ovum seems to be less advanced in its differentiation than that of the Teacher-Bryce I, because the network of plasmodium in the former is distinctly less abundant than that of the latter andbecauseofthepresenceofagreatquantityofbloodintheTeacher-BryceI specimen and the lack of blood in the trophoblast of the Miller ovum.
As was stated above, no final conclusions can be made as concerns the development of the entire embryonic primordium in these five specimens, but one fact cannot be doubted, the presence of mesoderm in stages where the primitive streak is certainly not yet present. The origin of the mesoderm in these stages must stilbe regarded as unknown because ofthe insufficiently good preservation of the specimens.
We comenowtothediscussionofspecimensnotonlymoreadvancedbut much better preserved than those mentioned above. The earliest of these are the embryos Peters and Werner (Stieve, 1931). The latter was described only verybrieflyatthemeetingoftheAnatomischeGesellschaftinBreslauin1931. Its measurements and its form show that it is certainly younger than the embryo Peters; its preservation also seems to be better than that of the latter. It is the youngest fairly well-preserved human embryo so far obtained. AsIhavestatedabove,thefiveearlierspecimensdonotadmitthe study of the development of the primary mesoderm because of their poor preservation. The Werner embryo is the earliest specimen where it may be suggested that at least a part of the primary mesoderm might originate from theembryonicectoderm.
In the description of the Fetzer embryo (Fetzer-Florian, 1930, pp. 433-6) Ihavediscussedthepossibilityoftheexistenceofan areainthemost caudal part of the embryonic disc where the primary mesoderm is fused with the ectoderm(fig.3). Ihavesuggestedthatwe mighthaveherebeforeus an area where primary mesoderm originates (at least in part). This area is situated close behind the primordium of the cloacal membrane. To this idea I was led by Prof. J. P. Hill's observations of a similar area in a very young embryo of Hapalejacchus(seeFetzer-Florian,1930,p.433). Bythekindpermissionof Prof. Stieve I have had the opportunity of making a series of graphic re- constructions of transverse sections of the embryo Werner (the specimen is cut longitudinally), and I have used the area in question (where the ectoderm of the most caudal part of the embryonal shield is fused with the primary mesoderm and where, I suggest, primary mesoderm probably originates) for the determination of the median plane, and I have utilised also the primordium oftheconnecting,stalkthoughthisisstilveryindistinct.The existenceofthe primitive streak in this stage can certainly be excluded.
Also in the Peters embryo a fusion of the ectoderm of the caudal part of the embryonal shield with the mesoderm was suggested by Graf v. Speed (Peters, 1899, p. 111), but the author did not give a further explanation of this condition. In 1909 Grosser(loc.cit.pp.78-9)designatedthisfusionofthetwo embryonallayersasa"wenigdeutlicherPrimitivstreifen." Rossenbeck(1923), who has studied the Peters embryo very carefully and has published a series of photomicrographs of the caudal part of the embryo, expressed the following opinion:"...kdnnteman inderTatdarandenken,dassessichhierum den morphologischen Ausdruck von Entwicklungsprozessen handelt, wie sic...als charakteristisch fur die Primitivstreifengenese beim Menschen vielleicht angenommenwerdenkdnnen"(loc.cit.p.360). But,accordingtohim,this fusion could be due to the inundation of the mother by K-ions (the mother was poisoned with caustic potash) and for this reason he pronounced the Peters embryo as unfit for the solution of this question.
In the more advanced embryo Beneke (fig. 4), in which Strahl and Beneke (1910) have described a primitive groove and Strahl a head process, Rossenbeck called attention to the fact that in the region of the former there was a fusion not only between the ectodernm and mesoderm but also between the mesoderm andendodernl. Rossenbeekdoesnotbelieveinthepossibilityoftheexistence of a primitive groove either in the Beneke embryo or in the embryo v. Herff (A-.Spec,1896)thoughv.Speeddescribeditaspresent. AccordingtoRossen- beck, ve. Speed was misled by an artificial deformation, which is difficult to believe. Rossenbeek suggests that the primitive groove exists first in the Matter embryo (Streeter, 1919) and he explains the fusion of the embryonal layers in the region in question in earlier stages as "Reste cines urspriinglich soliden Zusammenhangs im Embryonalzapfen," and the mesoderm originating
from this region as a derivative of an originally solid " Mesoblastauskleidung " of the primitive chorionic cavity possibly present in the embryo SCH (loc. cit.p.375).
Rossenbeck's opinion was accepted later by Grosser and v. Mollendorff. The latter originally described a primitive streak in the embryo OP (1921b), but under the influence of Rossenbeck's opinion explained (in 1925) the fusion of the ectoderm and endoderm in the embryos OP and WO as a primary fusion of the germ layers or as possibly the primordium of the cloacal membrane.
InaseriesofpapersIhavetriedtosolvethequestionofthesignificanceof this fusion. The general results at which I have arrived are as follows:
IntheembryosWernerandPeterstheareaoffusionoftheectodermofthe caudal part ofthe embryonic shield with the primary mesoderm ismost prob- ablythehomologueofthecorrespondingareadescribedbyJ.P.HillinHapale
jacchus. Hereatleastapartoftheprimarymesodermoriginates. How exten- sive this part is or whether the fusion of the ectoderm with the mesoderm is primary or secondary, cannot be determined because of the lack of adequately preserved material of earlier stages. The existence of this area in later stages, as I have suggested in the Fetzer embryo and as J. P. Hill has suggested from theexaminationofaphotographofasectionoftheembryoWO (seeFetzer- Florian, 1930; Hill, 1932), must remain in doubt because of the imperfect preservation of the Fetzer embryo and because v. Mbllendorff did not describe it in the embryos OP and WO.
IntheembryoOP (fig.1),thefirstprimordiumoftheprimitivestreakisto be found in the fusion of the ectoderm with the endoderm near the end of theembryonicshield. Nothingisknownabouttheformationoftheprimary mesoderminthisspecimen,aswasjuststatedabove. v.Mollendorffhas describedanareabehindtheembryonicshield(intheamnionaccordingtothe author) where the limits between the ectoderm and mesoderm are indistinct andtheectodermcelsareinprocessofdissolution,butthisareahasprobably nothing to do with the formation ofthe primary mesoderm, although itcannot be,inmyopinion,quiteexcluded;itssignificancewillbediscussedlateronp.274.
Similar conditions with regard to the primordium of the primitive streak canbeobservedalsointheembryoWO (fig.2)andFetzer(fig.3).Butalready in the former I believe the first indications of the formation of the cloacal membrane can be found in the thickened endoderm area in the dorsal wall of the yolk sac underneath the caudal margin of the ectoderm of the embryonal shield, which v. Mollendorff has described as the primordium of the allantois (fig.2,CO.?). Thetwoembryonallayersarenotyetfused.IntheFetzerembryo (Fetzer and Florian, 1930), there is a distinct fusion of the ectoderm and en- doderminthisregion,accordinglyafurtherstepintheformationofthecloacal membrane. Thisfusionissituatedunderneaththecaudalmarginoftheshield ectoderm and is separated from the primordium of the primitive streak by an area in which the embryonal layers are quite distinct from each other. The primordium of the primitive streak, accordingly, does not extend up to the cloacal membrane. The primitive streak and the cloacal membrane are separatedalsointheBenekeembryo(FlorianandBeneke,1930-1),butinthis specimen (fig. 4) the development of the primitive streak is much more advanced:itnolongerpossessestheformofanode(asisthecaseintheembryo OP,WO andFetzer),butnowformsashortstreak,andtheformationof mesoderm ishereverydistinct.
Figs.1-8.Mediansectionsthrougheightyoung humanembryos.Thecranialendon theright,the caudalendon theleft:x 100. Ectodermblack;endodermlinedhorizontally,headprocess linedobliquely,primitivestreakandsecondarymesodermlinedvertically.Primarymesoderm dotted. l.= cloacalmembrane;linedverticallyandhorizontally. Hensen'sknotmarkedbycrosses. P.pl.= prochordalplate;linedhorizontallyanddotted.A= amnioticcavity.Y.8.= yolk- sac cavity.
Fig.1. Embryo OP v. Mollendorff. (Basedon fig.39,Florian,1928.)
Fig.2. EmbryoWO v. Mollendorff. Cl.?= thickenedendodermintheplaceoflatercloacal membrane. (Basedon fig.40,Florian,1928.)
Fig.3.EmbryoFetzer.Thearea caudallytothecloacalmembranewheretheformationofthe primary mesodermisbelievedtooccur, markedbyarrows. (Basedon fig.53,Fetzerand Florian, 1930.)
Fig.4. EmbryoBeneke. (Basedon fig.1,FlorianandBeneke,1930-31.)
Fig.5.EmbryoBi[ttman]I.(Basedon fig.1,Florian,1927;somewhataltered.)
Fig.6.EmbryoT.F. (Basedon fig.43,Florian,1928. Somewhatschematic.)
Fig.7.Embryo"Hugo,"Stieve. (Basedon fig.6,Florian,1930b.)
Fig. 8. Embryo Bi[ttmann]
It is very difficult to determine whether the primitive streak has reached the cloacal membrane in the embryo T.F. (Florian, 1928) (fig. 6) because of the thicknessofthesectionsandtheimperfectpreservationofthespecimen. In the embryo Bi I (Florian, 1927) (fig. 5), which represents a stage only slightly more advanced than the embryo Beneke and certainly younger than embryo T.F., the primitive streak does not reach the very distinct primordium of the cloacal membrane, but the distance between the two is much smaller than in the embryo Beneke.
Grosser described a new young human embryo at the meeting of the Anatomische Gesellschaft in Breslau (1931 b), which belongs to a stage between theembryosT.F.andHugo. Inthisspecimenthereisadistinctfusionof ectoderm and endoderm caudally to the primitive streak and separated from thelatterbyanareawherethecolumnarcellsoftheectodermoftheembryonal shieldaredistinctlydelimitedfromtheunderlyingmesoderm. Inmy opinion, this fusion of the ectoderm of the embryonal shield with the endoderm repre- sents the primordium of the cloacal membrane. There is nothing strange in the separation of the primitive streak from the cloacal membrane, as this is the caseinthenextearlierspecimens. Grossersuggestedthattheprimordiumof thecloacalmembrane issituatedclosetothecaudalendoftheprimitivestreak in this specimen, but he admits that no fusion of the ectoderm and endoderm existsinthisplace.ThefusionwhichIinterpretasaprimordiumofthecloacal membrane in this embryo would be, in Grosser's opinion, a fusion of the amniotic ectoderm-n with the endoderm (of the yolk sac), therefore a structure which would be comparable only with the "canalis amnio-allantoideus." Tim existence in Man of a structure (fusion of the ectoderm of the amnion with the allantoic endodermn) comparable with the so-called "canalis amnio- allantoideus" was first suggested by Ingalls (1918).
In later stages the primitive streak reaches the cloacal membrane (this is certainly the case in the embryo Bi 24 (fig. 8) and probably also in the embryo Hugo)andbothstructuresbecomecontinuous. (IntheHugoembryo(fig.7) the primordium of the cloacal membrane was not described by Stieve in the original paper, but by the kind permission of Prof. Stieve I have been able to study the sections of this embryo and have seen the cloacal membrane; see Florian, 1928, p. 545.)
In the stage of the embryo Bi 24, the cloacal membrane first exhibits the form of an actual membrane, whilst in the earlier stages it is represented by a cell cord varying in length and connecting the ectoderm with the endoderm. In alwell-preserved embryos belonging to more advanced stages the cloacal membrane is distinctly present (embryos Schd, Waldeyer, 1929; Wa 17, Grosser,1931a;Kl13,Grosser,1913;Ho,Fahrenholz,1927;Dobbin,J.P.Hill and Florian,1931aandb;Peh.i-Hochsteter,Rossenbeck,1923,etc.).
Thestructureinterpretedbyme astheprimordiumoftheprimitivestreak in the embryos OP, WO, Fetzer and Beneke, as stated above, has the form of a node. The Beneke embryo is the earliest specimen in which it becomes distinctly elongated (towards the caudal end( of the embryonal shield) and so deservesthename"streak." Inalmoreadvancedspecimens(beginningwith the embryo Bi I)itselongated forni becomes stilmore distinct.
Just after the formation of the mesoderm has started in the primitive streak (embryos Beneke and Bi I),probably mostly by cell migration from the ectoderm, the mesodermal cells gather underneath the caudal extremity of the primitive streak; in the embryo Beneke, we (Florian and Beneke, 1930-1) could trace the secondary mesoderm behind the caudal end of the primitive streak around the cloacal niembrane into the connecting stalk. This was made possible by the fact that the secondary mesoderm in this early stage of its development was formed by very closely arranged cels, whilst the primary mesoderm possessed more the character of mesenchyme, its cells being separated from each other by more or less enlarged intercellular spaces. This observation is of importance, since it shows that the secondary or primitive streak meso- derm is not entirely used up in the formation of the embryonic body but certainly forms also a part of the connecting stalk. There is no evidence, how- ever,thatthesecondarymesoderm inMan couldbetracedup intothechorionic membrane.
The above-mentioned accumulation of the mesoderm cells underneath and behind the caudal extremity of the primitive streak leads to the formation of a node in this place, the "end-node" (Endknoten, Florian, 1928), observed alreadybyStieve(1926)iltheembryoHugo anddesignatedbythisauthoras "Siclhelknoten."Thisstructureistoberegardedastheproductoftheprimitive streak(seealsoStieve,1926,1).352). Itispresentonlyintheearlystagesof thedevelopmentoftheprimitivestreakandverysoon becomesindistinct,the cellsofwhichitiscomposedassumingthecharacterofmesenchiyme. Sowecan understand why it was not observed in more advanced embryos than the "Hugo." IntheBi24theend-nodeisstilrecognizable.
Ifwe recapitulate the chiefobservations stated above, we may distinguish three different areas in the caudal half of the embryonal shield in its median axis: (1) the most caudal area, differentiated in the earliest stages (embryos Werner,PetersandperhapsOP,WO andFetzer),inwhichatleastapartof the primary mesoderm is formed by cell emigration from the ectoderm of the most caudal part of the embryonal shield or close behind its caudal margin. (2) The intermediate area, i.e. the cloacal membrane, separated in the early stages (embryos Fetzer, Bi I, T.F., Grosser, 1931b), from the primordium of (3) the most cranial area, i.e. the primitive streak, which appears just before the developmentofthecloacalmembrane(embryosOPandWO). Itisnotpossible to determine whether the areas (1) and (2) are separated from each other or not becauseofthelackofadequatematerial. Ifwerealisethatthecloacalmem- brane and the primitive streak represent parts of the blastopore in lower vertebrates, we must accept the view that the homologneofthe blastopore in Man originatesintwo separated areas which grow together only during later development. This fact has perhaps caused the difficulties in interpretation of the structure which I regard as cloacal membrane. If we remember that the developmentofthecaudalpartofthehuman embryoisdistinctlyaccelerated in comparison with the cranial part (Florian, 1930a), we can understand the extremely early appearance of the cloacal membrane. The precocious develop- ment of the allantois and of the umbilical stalk is another proof of this inter- pretation. SoalsoisthefacttowhichG.Politzerhascalledmy attention,viz. that the caudal part of the human embryo becomes elevated above the level of the embryonal shield simultaneously with the cranial end, whilst in lower Mammals theelevationofthecranialendprecedesthatofthecaudalend.We may accordinglyregarditasacharacteristicofearlyhuman developmentthat the differentiation of the caudal part of the embryonal shield is accelerated, and that in the early stages certain areas of the embryonal shield (e.g. the region of the cloacal membrane) attain a development which in lower Mammals
is reached only in much more advanced stages. Itisworthyofconsiderationwhetherwe may interpretalsothearea(1) as a part ofthe blastopore, but there are not enough human embryos ofthis stage known to solve this problem finally. This area is certainly in no connection with the earliest primordium of the primitive streak, but, on the other hand, itisincloseproximitytotheprimordium ofthecloacalmembrane which isthe homologue of the ventral lip of the blastopore.
There seems to be a distinct difference between the development of the primary mesoderm in Man in the region in question and the development of the connecting stalk and the exocoelom in lower Primates (Tarsius). In Tarsius there is a structure developed which may be compared with the connecting stalk in Man (Hubrecht, 1902; Hill, 1932), and the exocoelom appearsinveryearlystagesasinMan andmuchearlierthaninotherMammals. But in Tarsius these two structures originate certainly in a formation which becomes a typical primitive streak (as Hill (1932) hasshown;Hubrecht(1902) has designated as "ventral mesoderm" what other authors cal primitive streakmesoderm), whilstinMan the site of origin of the primary mesodermis situatedbehindtheprimitivestreak,ifmy interpretationoftheconditionsin the embryo Fetzer is right.
Hill'sobservationonanearlyHapaleembryo(Hill,1932)representsavery importantcontributiontoourknowledgeoftheoriginoftheso-calledprimary mesoderm. There is an extreme similarity between this specimen (loc. cit. Plate9,fig.56)andtheWernerembryo. AccordingtoHill,wehaveinthe Hapale not to do with a typical primitive streak but with a structure which precedes the latter; with this explanation I am in complete agreement. The sameis,inmy opinion,thecaseintheWernerembryo. AsHillcouldstudy only one specimen of Hapale, it is not possible to determine whether the fate of the region in question in later stages is the same in Hapale as it is in Man.
We nowpasstothediscussionofonefactwhichmakesthestudyofthe development of the mesoderm and the distinction of the primary mesoderm from the secondary mesoderm very dificult. In very many places the limits between the mesoderm and the yolk-sac endoderm are so indistinct that the delimitation of these two layers is most difficult or quite impossible. There is evidence of fusion of the endoderm with the mesoderm of the primitive streak (at least in the median line), so that the former does not represent a distinct andindependentlayer(seealsoRossenbeck'sstatementsconcerningtheBeneke embryomentionedinthepaperof1923onp.365). Soitisintheearlieststages of the primitive streak before a distinct mesoderm formation takes place (see theprimitivestreakprimordiumintheembryosOP,WO andFetzer)and duringacertainperiodaftermesodermformationhasstarted. Ihaveseen this fusion of the endoderm and mesoderm in the embryos Beneke, Bi I, T.F. and Bi 24. Stieve has mentioned it in the embryo Hugo (1926, p. 355). The relationsaresimilartothosebetweentheendodermandwhatHillandFlorian (1931b) regard as mesoderm of the primitive streak accompanying the head processonbothsidesandcraniallycomingintoconnectionwiththeprochordal plate. Stieveinterpretsthisfusionbetweenthemesodermandendodermasa proof of mesoderm production by endoderm and calls this mesoderm " Dotter- sackmesoderm." The fusion can also be observed in many places far from the medianline. Fahrenholzhasdescribed(1927)asemicircularareainfrontof the head process in the embryo Ho, where the mesoderm is present and fused with the endoderm. Whether the "Dottersackmesoderm" really is the result ofmesodermformationfromendoderm,cannot,inmy opinion,bedetermined in sections, as there isstilthe possibility ofa secondary fusionofthetwo embryonal layers.
Theembryonallayersareinnoplacecompletelyseparatedfromeachother byemptyclefts,butareconnectedbymeansofcytodesmata. Intheearliest stages they are most distinct between the ectoderm and mesoderm of the amnion and the endoderm and mesoderm of the yolk sac. The ectoderm of the embryonalshield(apartfromtheregionoftheprimitivestreak,cloacalmem- brane and the region of the formation of the primary mesoderm) is in early stages very well delimited on its under surface, often by a very distinct basal membrane (visible even in badly preserved specimens). The prolongations of theunderlyingmesodermcelscanbetracedtothismembrane. Inlaterstages the celsofthe ectoderm ofthe embryonal shield become connected with the underlying mesoderm cels by means of distinct cytodesmata and the basal membraneisnolongerdistinguishable. (Fordetails,seeStudnilka-Florian, 1928, and Studnicka, 1929; the latter has studied the cytodesmata between the embryonal layers in a large series of human embryos.)
In the shape of the yolk-sac and in the form of its endoderm great differ- encesaretobeobservedeveninveryclosestages(cf.theembryosBenekeand Bi I, figs. 4 and 5, belonging to almost the same developmental stage). The yolk-sac process, originally described by Strahl and Beneke (1910), has been observed(inveryvaryingform)inmany otherspecimens.Thereisasyetno consensusofopinionamongstauthorsastoitsdevelopment.Thedevelopment oftheyolk-sacendodermitselfisquiteunknown.We donotknowwhetherthe yolk-sac arises as a closed vesicle or as a solid mass of endoderm, though Hill (1932) has brought forward evidence that in Selenka's Keim S of Nasalis larvatus the yolk-sac arises by the vacuolisation of an originally solid mass of endoderm. On the other hand, Stieve (1931) has advocated the view that theyolk-sacinMan isformedbythespreadingofanendodermallayerasin lower Mammals.
In regard to the yolk-sac endoderm, I desire to call attention to one fact: the endoderm underneath the centre of the embryonal shield (in front of the primordium of the primitive streak) in the embryos Fetzer (Fetzer and Florian, 1930), Schlagenhaufer-Verocay (1916, p. 164) and Bi I is composed of much enlarged,vesicularcels. IntheBenekeembryo(FlorianandBeneke,1930-1), there isalso such an area of vesicular cels, but itisfar in front of the cranial end of the primitive streak. The significance of this area is not yet determined (for further details see Hill and Florian, 1931 b). Finally we may mention the presence of cells in the most caudal part of the yolk-sac endodermn in the embryo Bi 24, which seem to be identical with those which Politzer has desig- natedasprimarygonocytes(Florian,1931). Ifthesecellsarereallyprimary gonocytes, the embryo Bi 24 would be the earliest human embryo, in which they have been recognized, as in the next younger embryo (Hugo) they are not present.
v. Mdllendorff called attention (1921b) to the presence of the "cell debris" (Zelltrimmner) situated in the amniotic cavity of young human embryos, and showed that they originate in a "Zerfallende Epithelwucherung" of the amnioticectoderm. Inhisopiniontheanmioticcavity(originatingbyde- hiscence)increasesinsizeatthecostofthis"Epithelwucherung." Ihavetried to show (1930 a) that the stalk ectoderm (i.e. the continuation of the ectoderm of the amnion in that region where it covers the mesoderm of the connecting stalkandwhichcorrespondsinveryyoungstageswiththe"Epithelwucherung" described by v. M6llendorff) invades the mesoderm of the connecting stalk also in much more advanced stages (for instance in the embryo Bi XI with ten paired somites), and that it can eventually separate off pieces of stalk meso- derm which come to lie free in the amniotic cavity where they are finally dissolved. In the connecting stalk itself, I have distinguished two parts: the amnio-embryonalstalkandtheumbilicalstalk. Inthesamepaperandpartly in a paper published by V61ker and myself (1929), it was suggested that the structure which was designated by other authors as "allantois" in early human embryos, really represents also a part of the hind-gut and should be called, therefore, " allanto-enteric diverticulum."
In this brief review I have dealt chiefly with the early development of the axial structures of the embryonal shield. For the accurate study of these structures the most important condition is the determination of the median plane,whichisonlypossibleafterthedeterminationofthesectionalplane. I have had the opportunity to determine the sectional plane in the embryos Fetzer (Fetzer and Florian, 1930), Beneke (Florian and Beneke, 1930-1), Bi I (Florian, 1927), T.F. (Florian, 1928 and 1930b), Hugo (Florian, 1930b) and Bi24. IhavealsotriedtodeterminethesectionalplaneintheembryosOP andWO (Florian,1930b),butmyestimationcouldbeonlyveryapproximate, because I have had no opportunity to study the sections and the original description of these embryos does not contain sufficient figures to permit a graphicreconstruction,suchasIcouldmake inthecaseoftheembryo "Hugo." The specimens referred to form a continuous series and make possible a detailed study of the early development of the axial structures.
It is not necessary to follow here the later stages in which the head process and the prochordal plate are established, since Hill and Florian (1931 b) have provided a review of the development of human embryos in these stages, with references to the literature.
In conclusion, I desire to thank Prof. J. P. Hill for his great interest in my work, for his help and for reading the manuscript of this paper.
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Reference
J Florian The Early Development of Man, with Special Reference to the Development of the Mesoderm and Cloacal Membrane. J. Anat.: 1933, 67(Pt 2);263-76 PMID 17104422
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