Paper - An Early Human Embryo, with 0.55 mm long Embryonic Shield

Embryology - 6 May 2024        Expand to Translate
Google Translate - select your language from the list shown below (this will open a new external page)
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt    These external translations are automated and may not be accurate. (More? About Translations)

Johnston, T. B. 1940. An early human embryo, with 0.55 mm. long embryonic shield. J. Anat, 75, 1-49.

Historic Disclaimer - information about historic embryology pages

Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

An Early Human Embryo, with 0.55 mm long Embryonic Shield

By T. B. Johnston

Guy's Hospital Medical School

Despite the fact that our knowledge of the normal development of the human embryo in its early stages has increased very substantially in the past twenty years, there are still many points on which further evidence is necessary and which cannot be elucidated until a much larger amount of material is available. The embryo on which this communication is based is a well-fixed specimen, and on this account a description of its main features may possibly contribute a little towards the solution of some of the outstanding problems. The embryo represents a slightly later stage of development than the beautifully preserved Strahl-Beneke (1910) specimen, and a slightly earlier stage than the Embryo Hugo (1926), with both of which it will be compared specifically. It corresponds in many ways to Grosser's (1931b) Embryo H.Schm.10,butas,at the time of writing, node tailed description of thisembryo has been published and only the figure of a schematic median section is available, I can use it only occasionally for purposes of comparison. The subject of this communication will be referred to as H.R. 1, as it was obtained at operationbyMrE.HeskethRoberts,towhom Iamverygreatlyindebted for the specimen and for the following clinical notes on the case.

History

The specimen was obtained fromtheuterusofawoman onwhom hysterectomy was performed. She attended the Gynaecological Clinic of St John's Hospital, Lewisham, in February 1932, complaining of pain in the right iliac and sacral regions. She was found to be sufferingfrom congestive dysmenorrhoea, with mild menorrhagia and marked leucorrhoea, which dated from her last confinement, two years previously, when she had given birth to healthy twins. Priortothatconfinementherperiodshadalwaysbeenregularevery 28daysandhadlastedfor5days,butsincethentheyhadincreasedinduration to 6 or 7 days, although the intermenstrual period remained unaltered. There wasnohistoryofpreviousmiscarriages. Onexaminationtheuteruswasfound to be moderately enlarged and the condition was diagnosed as either multiple small fibromyomata or chronic subinvolution, accompanied by an early stage of cystocele. It was decided to perform a subtotal hysterectomy, combined with abdominal cystopexy and she was admitted to hospital on 12 July 1932. The first day of the last menstrual period-was 17 June. The operation was performed on 14 July. A corpus luteum was found in the right ovary, and, when the uterus was opened after removal, the endometrium was found to begenerallyanduniformlythickenedandcongested. Itcontainedahaemorrhagic-lookingspotaboutthesizeofanordinarypin'shead.Themuscularwall of the uteruswasslightlythickened.Thewholeorganwasfixedin5% formalin and subsequently the haemorrhagic spot was removed for section together withthewholethickness of the underlyinguterinewall. A fewsectionswere cutandshowedthepresenceofanearlychorionicvesicle. Unfortunately,the blockwasmislaidforsometimeanditdidnotcomeintomy possessionuntil November 1935, when itwas no longer possible to obtain any further history orthedatesofcoitus.

Technique

It proved necessary to reblock the specimen. This was especially unfortunate as the chorionic cavity had already been opened, but the condition of the block left no option. The fresh block was cut into a perfect series of sections at 5,u, and the sections were stained with Mayer's haemalum and alcoholic eosin.

The sections were then drawn at a magnification of 200 and a reconstruction model was made in millimetre board.

The plane of section was rather unfortunate. At first I estimated that it made an angle of 100 with the cranio-caudal axis, but I am now of opinion that the angle is not much more than 50 so that the sections are nearly longitudinal (Text-fig. 1a). In addition, however, the section plane made an angle of approximately 570 with the horizontal plane (Text-fig. 1 b), and this obliquity has increased the difficulties of interpretation, especially at the caudal end of the shield.

The Ovum in Situ

Before the individual features of the embryo are described, a brief reference may be made to the appearance of a section through the whole ovum in situ. Such a section is shown in PI. 1, fig. 1. Theovum isnotdeeplyimplantedbut,liketheStrahl-Beneke(1910),the Embryo Hugo (Stieve, 1926) and many others, projects beyond the surface of the uterine mucosa. The amount of projection measures nearly 2 mm. beyondthelevel of the surroundingendometrium. An unusuallylargeblood clotcoversthecentralpart of the deciduacapsularisandobscurestheoriginal pointofentry.Thechorioniccavityisalmosttriangular in the section,withthe slightlybluntedapexfarthestawayfromthesurface of the mucosa. As usual, the embryo lies in the deepest part of the cavity and its connecting stalk is attached in the apex of the triangle, although this connexion is not shown in the figure.

The implantation is restricted to the stratum compactum, which shows numerous patches of localized oedema. One of the large venous sinuses, described by Bryce (1908), Falkiner (1932), Teacher (1924) and others, is seen deep to the decidua basalis, in the interval between the stratum compactum andthestratumspongiosum. Itiscutobliquely.Thestratumspongiosum shows the structure characteristic of an early pregnancy. The glands are enormously dilated and full of secretion, and their epithelium shows the familiar"saw-teeth"projections. A few of the glands in the neighbourhood of the ovum contain blood.

A fuller account of the endometrium will begiven in a subsequent communication.

Text-fig.1a. Schema of the dorsalprojectionofH.R.1.Thelineabisthemedianplaneandcd is the section plane. The angle y =5'. Text-fig. 1b. Schema of a transverse section through H.R. 1. The line ab is the median plane and cd is the section plane. The angle cof= 57.5°. The line c'd' represents approximately section20-10.

Reconstruction Model

As stated already, a reconstruction model of the embryo was made in millimetre board at a magnification of 200. It included the connecting stalk and a portion of the chorion but not the roof of the amnion, which had collapsed on the shield in places. The most striking feature of the outside of the model is the shape of the embryonicshield. Itis,asusual,moreorlessovalinoutline,butisconvex dorsallyinaldiameters. A slightdegreeofdorsalconvexityisnotuncommon in the early human embryo, but the degree of curvature is much greater in H.R. 1 than it is in the Bi. 1 (Florian, 1927), the T.F. (Florian, 1928), and the Thompson-Brash(1923)embryos. Itisamatterofconsiderableimportance and will receive full consideration at a subsequent stage.

A second, but less important, featureisthepresenceofathickenedrimof amniotic ectoderm round the cranial and lateral borders of the shield.

The median section of the model shows that the roof of the yolk-sac bulges up into the concavity of the ventral surface of the shield (Text-fig. 2). At first sight its cavity does not appear to be disproportionately small compared with the size of the embryo. This appearance is misleading and, if the shield were flat, or approximately flat, the small size of the yolk-sac, as witnessed by its measurements (Table 1), would be a very obvious feature.

Text-fig.2. Drawing of the mediansectionthroughthereconstructionmodelofH.R.1,partially idealized.Theroof of the amnionhasbeenremoved. x150.Thelinexyrepresentstheplane of the dorsal projection shown in Text-fig. 1a. A wide funnel-shaped diverticulum projects from the caudal surface of the sac and gives off from its summit a relatively long entodermal cord which passes into the connecting stalk (Text-fig.2). This solid cordofcellsisidentified as the primordium of the allantoic canal.

Hensen's node, which is seen in the median section of the model, lies caudal to the middle of the shield. Its position corresponds fairly closely to the position of the anterior end of the primitive streak in the Strahl-Beneke embryo, although, prior to the formation of the head- and tail-folds, it is usuallysituatedatoraboutthemiddle of the shield.Thepart of the primitive streakcaudaltothenodeisshortandexhibitsanearlystageofdifferentiation. It does not extend to the caudal limit of the shield. The head process is seen extending cranially for a short distance from the deep part of the node. Cranial to the head process the roof of the yolk-sac shows a patch of thickened entoderm (Text-fig. 2), and evidence will be brought forward to support the view that it represents the primordium of the prochordal plate. One other feature, visible on the median section of the model, should be mentioned at this stage. The entoderm of the distal end of the allantoic cord comesintocontinuitywiththeectoderm of the amnionimmediatelycaudalto the shield (Text-fig. 2). This represents the cloacal membrane (p. 88). The connecting stalk passes from the caudal end of the embryo to the chorion. Onitsrightside,closetoitsembryonicattachment,itismarkedby a deep groove, which is continuous at its right extremity with a tear into the amnion. This groove, owing to the nature of its walls, was regarded at first as a natural feature, but further consideration, prompted by a suggestion from Prof. Florian, led to the conclusion that it was an accidental tear. This discovery served to remove two dificultieswhich have not yet been mentioned. (1) The attachment of the allantoic cord to the yolk-sac lies considerably to theleft of the medianplane,aswillbeshown in the description of the individual sections,andthecorditselfshowsanangledbend. (2)Theyolk-sacisnot symmetricallydisposedwithreferencetothemedianplane,butprojectsmore totheleftsidethantotheright. The identification of the groove as an accidental tear made itclear that the embryonic end of the connecting stalk had become slightly kinked on the rest of the stalk and had swung the more ventral part of the yolk-sac over to theleft.

Brief Description of hte Individual Sections

The sections pass through the embryo from its left to its right side. Owing to the obliquity of the section plane (p. 2), the sections pass at first through the left and ventral walls of the yolk-sac, and it is not until the twenty-first section is reached that the embryonic shield appears. Owing to the angulation with the median plane (Text-fig. 1) the sections cut the median plane of the shield at its caudal end first. The individual slides are numbered serially and twelve sections are mounted on each.

Thefirstsectionthroughtheprimarymesodermontheleftwall of the yolk- sac appearsin16-4,butitisnotuntil16'10thatthecavity of the sacisopened into freely. The left edge of the amnion appears in 17-12, and the edge of the embryonicshieldin18-1. Sections16-4-17-12arenotfigured.

18-1(Text-fig.3). Theleftedgeof the embryo nicshieldandtheleftmargin of the amnionareseenbutthe amniotic cavity isnotyetopened. It lies in the figure to the left of the yolk-sac, which shows a short, wide diverticulum from theventralendofitscaudalwall(upperend in the photograph). Bloodislands are present on the ventral and cranial walls of the sac. The entodermal cells show a variety of forms.

18*8(Text-fig.4). The amniotic cavity isapparentandthecaudaldiverticulum of the yolk-sachasincreasedinlength. A fewstrandsofintra-embryonic mesodermareapparent. Inview of the conditionfoundinlatersectionsit should be noted that the primary mesoderm on the cranial end (lower end in the photograph) of the amnion passes straight on to the surface of the yolk- sac.

18x4 (Text-fig. 5). The shield has elongated, especially at its caudal end. The caudal diverticulum of the yolk-sacisalitlelongerandalitle narrower.

18-5 (Text-fig. 6). Both changes noted in the preceding section are progressing. In addition the intra-embryonic mesoderm forms an almost continuous layer over the caudal half or more of the embryonic area, and at its caudallimititscelsarecontinuouswiththose of the primarymesoderm. The celsliningtheyolk-sacareofatleastthreedifferenttypes: (a)on theventral wall they are cubical or low columnar; (b) on the cranial wall they form a syncytial ribbon; (c) in the roof they are elongated and flattened.

18-6 (Text-fig. 7). The caudal diverticulum of the yolk-sac is longer and narrower and shows a distinct bend, concave dorsally. The cels at its apex and on its ventral wall are large and cuboidal, while those on its dorsal wall aremoreelongatedandflattened. Dorsaltothediverticulumtheentoderm on the caudal wall of the yolk-sac shows an area of apparent thickening. This appearance is repeated in the ensuing four or five sections, and is regarded as beingduetoawrinkling of the wall of the yolk-sacassociatedwiththekinking duetothetear in the connectingstalk. A similarexplanationisofferedfor the additional cells which lie between the primary mesoderm and the caudal wall of the yolk-sac. The intra-embryonic mesoderm covers nearly the whole of the embryonic area, and consists of elongated cells with branching and anastomosing processes. 18-8 (Text-fig. 8). All the features noted in 18-6 are present, and the indications of wrinkling in the caudal wall of the yolk-sac are more marked. The intra-embryonic mesoderm establishes continuity with the primary mesodermatthecranialend of the shield. in this situation the exocoelom threatens to encroach on the embryonic area, and the encroachment becomes real in the succeedingsections. 18-9 (Text-fig. 9). At its caudal end the shield ectoderm and the amnion are cut obliquely, foreshadowing a further extension of the shield in a caudal direction. The part of the caudal diverticulum of the yolk-sac which lies distal to the bend already noted has become much narrower. At the cranial end of the shield the exocoelom encroaches into the embryonic area. 18-10(Text-fig.10). Theshieldhasextended farther in a caudal direction, and is cut obliquely at its caudal end; in this situation the shield ectoderm appearstobecontributingtotheformation of the intra-embryonicmesoderm. The narrow recess at the distal end of the caudal diverticulum of the yolk-sac hasjustlostitsconnexionwiththeinterior of the esac. in the cranialpartof the sac a group of entodermal cells is apparently lying free; they are part of a large wrinkle in the cranial wall of the sac which can be identified in the succeeding six sections.

18-11 (Text-fig. 11). A further extension of the shield in the caudal direction isapparent. A largepatchofcels,clearlyectodermalinorigin,liesventraland caudaltothecaudalend of the shield. Inpartthispatchrepresentsanoblique shaving through the shield ectoderm, but in part it appears to be forming intra-embryonic mesoderm. The recess of the caudal diverticulum of the yolksac has disappeared; its apex has given rise to a solid cord of entodermal cells which is cut transversely as it lies in the left part of the attachment of the connectingstalkto the embryo . Well-markedwrinklesarepresent in the cranial and caudal walls of the yolk-sac. The encroachment of the exocoelom on the embryonic area is quite definite.

18-12(Text-fig.12). Furtherextension of the shieldinacaudaldirection is indicated by the oblique shaving through the left wall of the ananion at the caudal end. The solid entodermal cord, which is regarded as the allantoic representative,isseencuttransversely,as in the precedingsection. Aboveit in the figure a few cells are visible; they are part of the connecting stalk, and the interval between them and the part which contains the allantoic cord is due to the bend associated with the tear in the connecting stalk.

19-1 (Text-fig. 13). Owing to an inequality of the floating out of the sections the curvature of the shield is less pronounced on slide 18 than it is on slide 19 and the succeeding slides. The shield continues to extend in a caudal direction, and the edge of the shield and the left wall of the amnion are again cutveryobliquelyatthecaudalendof the embryo . in this situationtwo groupsofcellsareseengrowingoutfromthedeepsurface of the ectodermand occupying the gap between it and the roof of the yolk-sac. Their significance isuncertain.Thecondition of the connectingstalkisunchanged. It still consists of two portions,one ofwhich (thelower in the figure)containstheallantoic cord. At the cranial end of the embryonic area the encroachment of the exocoelomisdeepand >-shaped.

19-2 (Text-fig. 14). No important changes are present in this section. A small,shallowdepressiononthedorsalsurface of the shield,whichispresent also in the succeeding section and then disappears, has no ascertainable significance. Two mesodermal cells are present in the > -shaped encroachment of the exocoelom.

19-3(Text-fig.15). The amniotic cavity hasextendedfurtherinacaudal direction and the caudal extension of the shield has almost reached its maximum. Abeautifulmitoticfigureispresent in the superficialpart of the shield near its caudal end. The two parts of the connecting stalk have now come together, so that the section passes through the floor of the bend on the left surface of the stalk. in this sectiontheroof of the amniotic cavity inits middle third shows an appearance almost identical with the appearance of the ventral part of the cranial wall of the yolk-sac in this and other sections. Itisimpossibletosatisfyoneselfthatitreally consists of two layersofcels for the twoappeartobefusedtogethertoformasyncytialribbon. Reference willbemadetothisappearanceatalaterstage. Atthecranialend of the shield the intra-embryonic mesoderm shows traces of cavity formation and it is impossible to be certain whether or not the spaces communicate with the exocoelom.

19*4(Text-fig.16). Nearitscaudallimittheventralsurface of the shield ectodermlosesitssharpcontour,whichissostrikinginitscranialtwo-thirds, and a small mass of cels is growing out from its deep surface in a caudal direction. The sections are now rapidly approaching the median plane, and thesecelsareregardedasderivatives of the end-node of the primitivestreak. Attention should be drawn to the irregularly <-shaped gap which separates the connecting stalk from the primary mesoderm on the dorsal surface of the caudaldiverticulum of the yolk-sac. A fewdetachedcellsoccupythegap, which represents the deepest part of the furrow described on the right side of the connecting stalk in the reconstruction model. The gap is more obvious in the later sections and has already been interpreted as an accidental artefact. At the cranial end of the shield the intra-embryonic mesoderm contains further and more obvious signs of cavity formation. A large blood island is present on the ventral wall of the yolk-sac. 19-5(Text-fig.17). The shield has now reached its full extent at its caudal end, and it is to be noted that in that situation the shield ectoderm consists of only a singlelayerofcuboidalcels.The endnode of the primitivestreakis seen0O08mm. fromthecaudalend of the shield. Itsconstituentectodermis thicker than the ectoderm of the rest of the shield and its ventral border is irregular, showing no trace of a basement membrane. There is no break in the continuity of the underlyingentoderm. Fromthecaudalend of the nodecells aregrowingcaudallyintotheconnectingstalk,thetearinwhichisnow more obvious. The allantoic cord is now cut obliquely as it bends towards the sectionplane. Atthecranialend of the shieldtheintra-embryonicmesoderm shows extensive cavity formation and its continuity with the primary mesoderm is again obvious. The exocoelom no longer encroaches on the embryonic area. A smallvascularspace,bestseenin19-6,ispresent in the bloodisland on the ventral wall of the yolk-sac.

19-6(Text-fig.18). Atitscaudalendtheshieldisnowreducedtoasingle layer of flattened cels, over an area 005 mm. long. The end node of the primitivestreakand thecels which it contributestotheconnectingstalkare clearly seen. The allantoic cord is now cut in its long axis, or nearly so. As before,itsoutlineissharpexceptatitsventralendwherethereisan appearance which suggests active growth towards the ectoderm in the anglebetween the amnion and the extreme caudal limit of the shield. This is the first indication of the cloacalmembrane. Atthecranialendalargecavityisnow apparent in the intra-embryonicmesoderm. Craniallyitswallsmeetand become continuous with the primary mesoderm. This cavity is regarded as a precociouscoelomicformation. Caudaltoitasmallpatchofthickenedentoderm is adherent to the basement membrane of the shield ectoderm.

19*6(PI.1,fig.2). Theprecociouscoelomiccavityandtheadjoiningparts of the embryo areshownatamagnificationof950. Itissignificantthat whereastracesofcytodesmataarevisibleconnectingtheonewalltotheshield ectoderm and the other to the roof of the yolk-sac, there is no evidence of any such connexions across the cavity itself. With the exception of one rounded celalthecels in the walls of the cavityareelongatedand typical mesoblasts.

19-7(Text-fig.19). Theflattenedcelsatthecaudalend of the shieldare shrunkenand in the underlyingmesodermasmallgapindicatesacontinuation of the tear which in the ensuing sections breaks through into the amniotic cavity. The allantoic cord is again cut in its long axis in the connecting stalk. Its apex and caudal (right-hand) border are sharply delimited from the overlying primary mesoderm, but its cranial (left-hand) border, especially in its ventral (lower)part, isvery indefinite,and in thisregion the entodermal cels are in continuity with the celswhich are interposed between them and the amniotic cavity and which are regarded as a thickening of the amniotic ectoderm. This ecto-entodermal continuity is identified as the cloacal membrane. In the dorsal and cranial (upper and left-hand) part of the connecting stalk aroundedmassofcels,suggestiveofalargebloodisland,alsorepresentspart of the ectodermal constituent of the cloacal membrane, as will be apparent in thesucceedingtwosections. Cellsderivedfromtheendnode of the primitive streakarestilapparentand,slightlynearertothecranialend,alargemassof cels is growing headwards from the primitive streak. This mass, which is 0*03mm. long,seemstobedevoidofentodermonitsventralsurface. Itis equallyconspicuous in the nextsection,scarcelyrecognizable in the nextbut one,andthereafterdisappears. Cranialtothisoutgrowththeshieldisslightly thickened and its ventral surface is irregular and woolly. This represents the firstshavingthroughtheleftborderofHensen'snode. Atthecranialendof the shield the precocious coelomic cavity is again conspicuous, and caudal to it the entoderm of the yolk-sac is adherent to the basement membrane of the shield ectoderm, although no thickening is apparent. The presence of a desquamated entodermal celshould be noted lyingfree in the yolk-sacbelow themiddle of the shield.


19*7 (Text-fig. 20). The outgrowth from the primitive streak and the adjoiningshavingthroughtheleftborderofHensen'snodeareshown x684. The outgrowth is obviously directly continuous with the ectoderm of the streak.

19*8 (Text-fig. 21). The tear at the caudal end has now passed through the shield into the amniotic cavity. The allantoic cord isnot so easy to recognize. Itscaudalborder(rightborder in the figure)isstilsharpexceptatitsmiddle, but its ventral border becomes directly continuous with the adjoining cels, whichareectodermalinorigin,derivedfromtheadjoiningpart of the amnion. Thisisundoubtedlythecloacalmembrane.Theroundedmassofcelsobserved in the dorsal and cranial part of the connecting stalk in the preceding section is continuous ventrally, in this and in the next section, with the ventral part (lowerpart in the figure) of the cloacalmembrane. Atitsnarrow,caudalend theappearancesuggestscontinuitywiththeapicalpart of the allantoiccord, andthisappearanceisconfirmed in the nextsection. in this section,therefore, the connecting stalk, with the exception of its covering layer of primary mesoderm, consists almost entirely of the cloacal membrane. The end node of the primitive streak is no longer visible, but the outgrowth from the streak inaheadwarddirectionisagainapparent. Atitscranialendthisoutgrowth reaches the caudal surface of Hensen's node, which now involves the whole thickness of the embryonic area. The sharp ventral contour of the shield ectoderm stops very abruptly at the cranial limit of the node. The characteristic structure of the node is seen better in sections 1910 and 1911. The condition of the roof of the yolk-sac immediately cranial to the node is difficulttointerpret. Itisformedbyanumberoflowcolumnarcellswhich appeartobecontinuouscaudallywiththedeeppart of the node. Attention should be drawn to a large, round entodermal cel,with a deeply pycnotic nucleus, which is lying free in the yolk-sac in this situation, and to the fact thatmorecraniallytheentodermseemstostopabruptlyatacellwhichshows a good mitotic figure. The low columnar cells are regarded as forming the left edge of the primordium of the headprocess. Betweenthemandtheectoderm a group offivemesodermal celsisinterposed.These celsreallylieto theleft of the headprocessbutappeartoliedorsaltoitowingtotheobliquity of the section plane. At the cranial end of the shield the coelomic cavity, though present, is in two parts and is not so convincing as it is in the two preceding sections. Caudaltoittheentodermisstiladherenttotheshieldectoderm, althoughthereisno localizedthickening.

19-9(Text-fig.22). in the connectingstalktheentoderm of the allantoic cordisnowcutintwoplaces.Thelarger,proximalportionformsa >-shaped, darkly stained strip, with the concavity facing the amnion. The concavity of the> isoccupiedbytheectodermalthickening of the amnion.Thedistal part of the cord,alsodarklystained,isdirectlycontinuouscraniallywiththe large group ofround celsnoted in the previous section. The outgrowth from the primitive streak immediately caudal to Hensen's node isstilpresent but it is much smaller. The node itself is larger, as the sections at this level are close to the median plane. The roof of the yolk-sac cranial to the node is formed by a number of low columnar cels, which are not so obviously in continuitywiththenodeowingtothepresenceofashrinkageinterval. Their nuclei are large and darkly stained, resembling in every way the nuclei in the node.Theyareseparatedfromtheshieldectodermbyagroupofthreeslightly elongatedmesodermcelswhichalsoappeartobederivedfromthenode. At thecranialendtheprecociouscoelomiccavityisagainpresentandwellformed. Caudaltoittheentodermisagainadherenttotheshieldectoderm. in this situation it is slightly thickened and consists of a number of large, vesicular cellswithroundnuclei.

19-10(Text-fig.28). Most of the connectingstalkisstiloccupiedbythe remains of the cloacal membrane, but in its caudal part there is a greatly increasednumberofmesenchymalcels. Hensen'snodeiscutapproximately initsmedianplane. Itinvolvesthewholethicknessof the embryo nicarea and no entodermal celsarepresenton itsventralsurface.The pallor of the nodeisverystriking.Thisisdueinparttothefactthatitsnucleitendtobe grouped into oblique, overlapping rows, leaving wide areas of non-nucleated cytoplasm,andinparttothefactthatthecytoplasmstainsratherfaintlyand containsshrinkagespaces. Fromthecranialsurface of the deeppart of the nodetheprimordium of the headprocessextendscranially. Itnow consists of five narrow, columnar cels which form the roof of the yolk-sac and extend almosttothebasementmembrane of the shieldectoderm. A shrinkagegap separatesthemost cranial of the secelsfrom two smallercels,obviouslyof the same character, although they are not typically columnar in shape. The nucleiofalthese cels of the head process staindarkly, showing a prominent nucleolus, and are identical in appearance with the nuclei in the deep part of thenode. Fromthecaudalsurface of the nodetypicalmesodermalcelsgrow caudally between the ectoderm and the entoderm. A single, desquamated entodermal celispresent in the cavity of the yolk-sac ventral to the node. Atthecranialendof the embryo theprecociouscoelomiccavityisherereduced toaslitwithslightlythickenedwalls. Caudaltoittheentodermshowsa large thickened patch of vesicular cels and is again adherent to the shield ectoderm. This is regarded as the primordium of the prochordal plate and will bereferredtounderthatname.

1911(Text-fig.24). Thelasttraces of the cloacalmembranearevisiblein the connecting stalk. Hensen's node and the head process are readily recognizable. From the caudal surface of the node mesodermal cels are extending caudally between the ectoderm and the entoderm. Two desquamated entodermal cels lie free in the yolk-sac (below the caudal part of the node in the figure). Atthecranialendof the embryo thecoelomiccavityisobscured, but caudal to it the prochordal plate is a conspicuous feature.

1911(P1.2,fig.1).Hensen'snodeisshown x950.Thewideareasofnonnucleated cytoplasm and the crowding together of the nuclei are well shown. Thenodecontainsamitoticfigurenearitscentre.Theheadprocess consists of fivecolumnar cels,andamitoticfigureispresentonitsventralsurfaceatits nodalend. Cranialtothesecolumnarcelstherearethreesmallercels,whose nucleishow thesame stainingafinitiesasthenuclei of the node; theyprobably represent the headward end of the process.

19-12(Text-fig.25). Caudallythetear in the shieldhasledtothesagging away of the connecting stalk. The cloacal membrane has disappeared, and a massofmesodermalcelsoccupiesthesiteofitsmoreventralportion. Hensen's node,whichisdividednearitsrightborder,isveryconspicuous. A shrinkage gapoccursatthecranialend of the headprocess.Thestructure of the nodeand headprocessareshown x950inP1.2,fig.2. Atthecranialendof the embryo the intra-embryonic mesoderm is in process of cavity formation, and the ventralcels-showthe"syncytialribbon"appearancealreadynoted in the roof of the amnion and the cranial wall of the yolk-sac. The prochordal plate standsoutprominently;itsdetailedstructureisshown x950inPI.3,fig.1.

19-12 (PI. 2, fig. 2). The nuclei in Hensen's node are arranged in two oblique, overlapping rows, directed cranially and ventrally. They are not remarkable for their size. The head process consists of three or four columnar cels which occupy nearly the whole space between the shield ectoderm and theroof of the yolk-sac.Thereisobvioustissuecontinuitybetweenthecaudal end of the process and Hensen's node.

19-12 (PI. 3, fig. 1). The prochordal plate consists of about ten large, vesicular,entodermalcels. Theirnucleidonotstaindarklyandappearoval in this section. The plate is adherent to the basement membrane of the shield ectoderm over a considerable area.

20-1 (Text-fig. 26). The mass of mesodermal cels seen in the connecting stalk in the precedingsectionisagainprominent. Onlythedorsalpartof Hensen'snodeiscut,asthesectionpassesclosetoitsrightborder. Cranialto ittheintervalbetweentheectodermandtheyolk-sacisoccupiedfor0*08mm. bycelswhichmay beregardedasformingtherightedge of the headprocess. At the cranial end of the embryo the intra-embryonic mesoderm contains a verydistinctcavity,whichislessextensive in the cranio-caudalaxisthanitis insome of the precedingsections. Itswallsarebeautifullyfixedandshowthe "syncytialribbon"appearance. Caudaltoittheprochordalplateisaconspicuousfeature.

20-1 (PI. 8, fig. 2). The prochordal plate is only adherent to the shield ectodermatonepoint. Itisofapproximatelythesamethicknessastheshield and comprisesfourteenorfifteenlargevesicularcelswithsphericaloroval nuclei,whichdisplayaprominentnucleolusandafinechromatinnetwork. A fine coagulum extends into the yolk-sac from the ventral surface of the plate.

20-2(Text-fig.27). Unfortunately in this sectiontheconnectingstalkhas floated away from the embryo and has become so wrinkled that its structure cannot be examined. The lasttrace ofHensen's node isseen as a palidarea in the mostdorsalpart of the shield. Cranialtoitmesodermalcelsfilup nearly the whole interval between the shield ectoderm and the yolk-sac, the roofofwhichisagainentodermal. Clearlythesecelslieontherightside of the head process and may belong to or may be derivatives from it. At the cranial end of the embryo the precocious coelomic formation appears for the last time.Theprochordalplateisnolongeradherenttotheectoderm. in this and in the precedingsectionthereisanextensivewrinkling of the wall of the yolksac where the roof becomes continuous with the cranial wall.

20-8 (Text-fig. 28). The sections have now reached the right margin of the caudal end of the shield. The angled recess at the dorsi-caudal end of the amniotic cavity isbeingpartiallyfiledup fromitsrightside,asthesectionis shaving obliquely through the amniotic covering of the connecting stalk. The recessdisappears in the ensuingsections.Theprochordalplateiseasilyidentified, but some mesoderm cels now intervene between it and the ectoderm. Elsewhereafalseappearanceofentodermalthickeningis presented in the roof of the yolk-sac. Thisisduetothefactthatthesectionsarenowrapidly approachingtherightwall of the sac,whichiscutobliquelyinseveralplaces. The wrinkling of the cranial wall at its junction with the roof gives rise to a very exuberant infolding in this section. The succeeding sections pass through the embryo with increasing degrees of obliquity and are entirely to the right of the medianplane. Asthemedianplanestructures of the shieldarenolonger divided, only selected sections from the rest of the series will be figured.

20-8 (Text-fig.29). This sectionshowsasubstantialshavingthroughthe rightwall of the yolk-sac,whichappearsasanislandofsolidentodermwithin theconcavity of the shield.Theconnectingstalk,nowapproachingthechorion, is looser in texture and shows the presence of a number of vascular spaces.

20-10(Text-fig.30). Thissection,afterdividingtheshield,passesthrough theintervalbetweenitsrightslopingsurfaceandtherightwall of the yolk-sac. The interior of the concavity of the shield is occupied by the branching, interconnected cels of the intra-embryonic mesoderm, and thegeneralcontinuity of the openmeshwork of the mesodermiswelldemonstrated. Inmanyplaces the shield ectoderm appears to be giving origin to mesoderm cels, but, owing to the obliquity of the section, too much stress must not be laid upon this appearance. A well-formedvascularspace,linedbyflattenedelongatedcels, isseen in the connectingstalk(lowerandleftpartinText-fig.34). Other,less convincing,spacesarealsopresent.

From this point onwards, owing to the curvature of the shield and the obliquity of the sectionplane,thesectionsthroughtheshieldectodermbecome more and more nearly tangential.

21-4(Text-fig.31). Thissectioncutsthroughtherightsurface of the shield tangentially so that it appears as a solid mass of ectoderm. The connecting stalk is very loose in texture and has almost reached the chorion. 21-10(Text-fig.32). Thissectionshavesthroughtherightsurface of the upturned rim of the amnion to the right of the right border of the shield. The connecting stalk is now in continuity with the primary mesenchyme of the chorion. Boththestalkandthecavity of the amnioncanberecognized in the succeedingsections.

Now that the reconstruction model and al the important individual sectionshavebeendescribed,itispossibletodiscussthesalientfeatures of the embryo and the problems arising from them.

The Embryonic Shield

In most well-preserved human embryos at or near the stage of development exhibitedbyH.R.I theshieldisflator verygentlyconvex dorsaly, and one is tempted to suggest that the curvature of H.R. I is the result of slightcompressionduringfixation. Suchan explanation,however,isinsufficient to account for the condition in its entirety. In the first place the shield is convex in both its longitudinal and its transverse diameters, and no degree of compression could bring about this double curvature without causing excessivetearingorwrinkling of the surface of the shield. Apartfromthe small tear at the caudal end, which is quite incapable of accounting for the condition,theshieldisintactandshowsnosignofwrinkling. in the second place the yolk-sac shows al the appearances that might be expected ifthe curvatureweretheresultofagrowthprocess. Itsdome-shapedroofiseverywhere adapted to the curve of the shield and shows no sign of wrinkling or tearing.

On thesetwogroundsIbelievethatthecurvature of the shieldistheresult of abnormal growth, though it is probable that it has been exaggerated to someextentduringtheprocessoffixation. Attentionhasalreadybeendrawn to the fact that the embryo was situated in the deepest part of the chorionic cavity,which in this caseisthenarrowestpart of the space. Itisnotunlikely that the confinement of the embryo in this restricted space may have helped toaccountforitsabnormalshape.

Onewouldnot,however,bejustifiedonthisaccountindiscountingentirely the importance of the evidence which this embryo offers with regard to other developmental problems, for abnormalities of shape are by no means uncommon. The shield of the Strahl-Beneke embryo is concave dorsally, while in the Bi. 1, the T.F. and the Heuser (1982) embryos it is convex like the shieldofH.R. 1,but,nevertheless,altheseembryoshaveprovidedimportant contributions to our knowledge of the normal development of the human embryo in its early stages.

The shield ectoderm consists of high columnar cels. arranged usually in three, though occasionally in four and sometimes in two, overlapping rows. Thecels, for the most part,aresetatrightanglestothefreesurface,butin many places their long axes are oblique (P1. 3, fig. 1), a fact which may be regarded as confirming the view that the shield curvature is the result of growthprocesses. Atitscaudalendtheshieldectodermisabruptlyreduced to a single layer of rather flattened cels over a length of 006 mm. This reduction in thickness is present in both the Strahl-Beneke and the Hugo embryos, and in both it extends as far caudally as the cloacal membrane. Further reference will be made to this feature when the primitive streak is discussed.

The cytoplasm of the cels of the ectoderm stains darkly, and the nuclei, whichareelongatedoroval for the mostpart,showarichchromatinnetwork andaprominentnucleolus. in the surfacelayernumerouscelsoccurwith enlarged,almostsphericalnuclei,thecytoplasmofwhichislessdarklystained. Thesemaypossiblybecelswhichareabouttoundergomitosis. Inmostof the sections mitotic figures are present, the cels concerned being, with few exceptions,situated in the mostsuperficiallayer. Onlyonewasfound in the caudal part of the shield, but three were observed in the depths of Hensen's node.Thisdistributionofmitoticfiguresagreeswellwiththeconditionfound in both the Hugo and the Strahl-Beneke embryos, and may be regarded as indicativeofrapidincreaseinsize of the pre-nodalpart of the shield. Itis clearalsothatenlargement of the shieldatthisstageisbroughtaboutbythe division of the cels of the superficiallayer.

Intracellular darkly staining droplets are a very striking feature in most of the sections (PI. 2, fig. 1). They vary considerably in size and are found at aldepthsfrom thesurface,beingmost numerous in the cranialtwo-thirdsof theshieldand in the regionofHensen'snode. Inplacesthedropletsrun together, forming crescentic figures, and they are usually surrounded by a smallareaoflightlystainedcytoplasm. Droplets,identicalinappearance,are found within the cavity of the amnion (Text-fig. 24), and their presence there in considerable quantity suggests that they may be of the nature of an excretion. They have been observed by Florian in the Embryo Fetzer (1929). No signs-ofdegenerativechangeswereseen in the superficialcels of the shield.VonMollendorif(1921a)describedsuchchanges in the Ovum O.P.and regardedthemasnormal. Stieve(1926)opposedhisinterpretation,andthe evidence available from H.R. 1 strongly supports his view.

Except in the regions of Hensen's node and the primitive streak, the deep surface of the shield ectoderm is sharply demarcated, and the appearance is stronglysuggestive of the presenceofabasementmembrane. Grafv.Spee (1896) was the first to describe the existence of such a membrane, and it has been identified more recently by von Mollendorff (1921 a), Rossenbeck (1923), Ramsey (1937),andothers,sothatitmay beregardedasanormalstructure. In H.R. 1 the shield shows the presence of many fine, obliquely vertical slits which are the result of shrinkage during fixation. These slits are visible in many of the figures, and it is to be noted that, although in some places they reach the basement membrane, they do not break through it. In von Mollendorff'sOvum O.P.thebasementmembranewaspartiallydetached,afactwhich Stieve(1926)regardsasindicativeofpoorfixation. Innosituationisthe basement membrane detached in H.R. 1. At the cranial border of the shield, and along each lateral border, the transitionfromtheshieldectodermtothetypical,flattenedepithelium of the amnion is gradual. As a result the shield appears to have a turned-up rim everywhereexceptatitscaudalend.Thisrimisregardedaspart of the amnion and has been excluded in estimating the measurements of the shield. It bears no relation to the entoderm of the yolk-sac, and the primary mesoderm passes continuously from its outer surface on to the cranial and lateral walls of the sac. A similarrimispresentalongthelateralmargins of the Fetzer embryo (Florian, 1933), and in the Strahl-Beneke the transition from shield to amnion at the cranial end is so gradual that it is difficult to determine the precisecraniallimitof the embryo nicarea.

Streeter (1936) has recently put forward the view that the first and most fundamental differentiation in the morula mass separates the cells which take part in the formation of the embryo itself from those which form only extraembryonicstructures. Amongstthelatterheincludesthecells of the amnion, and as evidence in favour of this view he cites the abrupt character of the transition from the shield ectoderm to the ectoderm of the amnion. This view will be considered later in connexion with the yolk-sac, but from a study of the embryos cited it is clear that the transition from shield ectoderm to amniotic ectoderm may be either gradual or abrupt, and that its character cannot be adduced as evidence in support of Streeter's view.

The Primitive Streak

The primitive streak, including Hensen's node, has been identified in sections19-4-202. Itmeasures0-115mm.inlengthanditscaudalendfals short of the caudal end of the shield by 009 mm.

The cranial half, or rather more, of the streak can readily be distinguished from its caudal half. It stands out as a relatively pale area in the darkly stained shield (PI. 2, fig. 2) and shows features which indicate that it may justlyberegardedasanearlystageofHensen'snode. Atthecranialendof the streak the sharp contour of the deep surface of the shield ectoderm ceases abruptly, and there is tissue continuity through the whole thickness of the embryonic area, the entoderm being absent from the ventral surface of the streak. The structure of the node has already been described and reasons have beengivenforitscharacteristicpallor(p.23). Inolderembryos,suchasthe Manchester embryo (Hill & Florian, 1935), the contrast between the pallor of the node and the dark staining of the adjoining primitive streak is sometimes very striking. No such contrast, however, is present in the figures of the Embryo Hugo or Strahl-Beneke, and a very similar pallid area in the Fetzer embryo is regarded by Florian as the primitive streak itself prior to node formation. No surface elevation marks the position of the node, but, although Stieve lays stress on its presence as a means of identification, it is clearfromEmbryo Bi.24thatatypicalnodemay bepresentwithlitleorno surfaceelevation. Inolderembryos,thesurfaceelevationismoreconstant. In identifying the pallid area in H.R. 1 as Hensen's node, I have been more influenced by the fact that it is giving rise to the primordium of the head process.

Theprimitivestreakproperisveryshort,measuringonly0055mm. long. It is in an early stage of differentiation and does not yet extend to the cloacal membrane, being separated from it by a zone of undifferentiated ectoderm, 009mm. long.Theconstituentcels of the streakarewiderthanthose of the rest of the shieldectodermandhavelargeovalnuclei.Theyarepackedclosely together and few shrinkage slits, such as have been described on p. 32, can be seen. The cytoplasm of the celsstainsdarklyand no difficultyisexperienced indeterminingwherethestreakendsandHensen'snodebegins. Atitscaudal endthedeepsurface of the streakgivesorigintoagroupofcels(Text-fig.18) which stream into the connecting stalk. These cells are, I believe, identical withthosedescribedbyStieveasformingthe"Sichel-knoten"in the embryo Hugo and by Florian as the "end node" in the Strahl-Beneke embryo. They must be regarded as characteristic of the early stages of the primitive streak, but no light has yet been thrown on their significance. The small mass of large ectodermal cels which springs from the cranial end of the streak proper (Text-fig. 20) is difficult to interpret, for the cells do not appear to be mesodermal, as they retain their ectodermal characters unchanged. The only approximately similar appearance which I have been able tofind in the literatureis in the Meyer(1924)embryo,whereitwasidentifiedwronglyaccordingtoFlorian(1928)-astheheadprocess. Ifthetwostructures are actually identical, Meyer's interpretation must be erroneous, for in H.R. 1 the mass lies caudal to Hensen's node.

The ventral surface of the streak proper is finely irregular and shows no trace of the presence of a basement membrane. Ventral to it the entodermal roof of the yolk-sacisintact. Owingtothesectionplaneitisimpossibletobecertainwhethertheedges of the streak proper are actively giving rise to intra-embryonic mesoderm or not. The subject will be dealt with later, when the intra-embryonic mesoderm isconsidered.

The Head Process

A shortmassofcels,broaderatitscaudalendthanatitscranialend,has been identified as the primordium of the head process in H.R. 1. The mass is clearly in continuity with the cranial'end of Hensen's node and its nuclei are identical, both in staining reactions and in appearance, with the nuclei of the node. Itisnotmorethan004mm.longandisthereforeinanearlystageof development. Exceptnearitscranialend,whichisdificulttodetermine precisely, the cels of the process are columnar and present a picture (Textfig. 28) which suggests that they may in reality be entodermal. That possibility, however, is excluded by the continuity of the process with Hensen's nodeandbythecharacterofitsnuclei. Further,with in the cavity of the yolksac in this region there are five desquamated entodermal cells (one each in 19*7, 19*8 and 19*10 and two in 19.11) in addition to some detritus in 19*12 whichmay representtheremainsofoneortwo entoderinalcels. Itissuggested that these are some of the cells which originally formed the roof of the yolk-sac in this situation and which have been replaced by the cels of the headprocess. in this connexionitshouldbeobservedthatin the embryo Fetzer,whichshowsno signofaheadprocess,thecels in the medianplaneof the yolk-sac roof immediately cranial to the primitive streak are vesicular in character.

Apart from the Embryo Meyer, which has already been discussed and excluded, the H.Schm. 10, the Hugo and the Bi. 24 embryos are the youngest which show the presence of an undoubted head process. In H.Schm. 10 a headprocess,0*1mm. long,isalreadypresentand inone sectionitcontains a space, lined dorsally by high columnar epithelium and ventrally by low columnar epithelium, which is identified by Grosser (1931b) as Lieberkfihn's canal. in the Hugoembryotheheadprocessis009mm. longandconsists ofan irregularlydefinedmass ofmesodermal cels,more compact at itscaudal endthanatitscranialend.Thecelsof which it consistsareincontinuitywith the intra-embryonic mesoderm on each side and with Hensen's node caudally; theyarefusiformorroundedinshape for the mostpart,butnowherearethey columnar. Inno human embryo yetdescribed,withtheexceptionofGrosser's H.Schm. 10, have any columnar cels been observed in the head process. According to Stieve the head process of the Embryo Hugo has no entoderm on itsventralsurface,butFlorianbelievesthattheentodermalroof of the yolksacisintact. Iam satisfiedthatthestructureinquestioninH.R.1isa derivative of Hensen's node, but it will be necessary to wait until other embryos of the same stage are available before one can be certain that it is the normal, early condition of the head process.

The cloacal membrane will be described with the entoderm (p. 38).

The Yolk-Sac

Attentionhasalreadybeendrawntothesmallsize of the yolk-sacandthis featurecanbeappreciatedbetterfromaconsiderationofTable1.Thereasons why itis masked in the reconstruction model and in the actual sections have been explained earlier. Too much stress should not be laid on the size of the yolk-sacasanindicationofnormaldevelopment,foralthoughthereisnaturally a distinct tendency for the yolk-sac to enlarge proportionately to the growth of the shield prior to the appearance of the headand tail-folds, there are many notable exceptions.

The entoderm lining the yolk-sac is remarkably varied in its structure. The roof, apart from the region of the prochordal plate and the head process, isformedofflattenedcelsandlow,cubicalepithelium.Theventralwall of the sacshowsthewidestrangeofvariation. Atonepoint(Text-fig.33)thereis asmallpatchoffiveorsixtypical,highcolumnarcels. A largeareaiscovered by low columnar epithelium, including some very large and obviously degeneratingcelswithsphericalnuclei,some ofwhicharehydropicand others intensely pycnotic. Low columnar epithelium forms the ventral wall of the caudal diverticulum in the neighbourhood of the origin of the allantoic cord (Text-fig.9). Elsewherethecelsareelongatedandslightlyflattened. In patchesthesecelsarealmostendothelial in the irappearance; oftentheyform what can only be termed a "syncytial ribbon". That the last two varieties are different forms of the same type is shown by their repeated occurrence in immediately adjoining areas, and by the fact that where the "endothelial" type occurs the cytoplasm is obviously coagulated and shrunken. The same two types of epithelium are found in adjoining patches in the Strahl-Beneke embryo, and are present, though not so conspicuous in the Embryo Hugo. The "syncytial ribbon" type appears to be better fixed than the "endothelial"type.

Inalembryostheventralwall of the yolk-sacshowsthegreatestrangeof variationasregardsthecharacter of the cels. In view of the varieties of epithelium to which the intra-embryonic entoderm gives origin in the course of development, the variations noted in H.R. 1 arenotaltogethersurprising. Nevertheless,itmust bestatedthatthecelsin the roof show less variation than the other walls, and this may prove to be of importance in connexion with (1) the view advanced by Corning (1925) and supported by Stieve (1936) on the evidence of the findings in the Embryo Werner; and (2) the view recently advocated by Streeter (1986). Coming's theorythattheyolk-saccavityisoriginallypart of the magma cavityoutlined inprimarymesodermandthattheentoderm of the roof,whichisderivedfrom theinnercelmass,graduallyspreadsovertheinterior of the cavity,isneither confirmed nor refuted by the evidence from H.R. 1. The possession of an obvious variety of potencies in the cels of the ventral wall of the yolk-sac might, on the other hand, be regarded as evidence in favour of Streeter's view (already stated), in accordance with which differences in the characters of the roofandtheotherwalls of the yolk-sacmightbeanticipated. Itmust,however,beemphasizedthatatthisstagethepotencies of the celsconcernedare apparentlynotyetmateriallyreducedandtheexhibitionofdifferenttendencies in different areas does not of itself necessarily imply differences in developmental origin. I regard the fact that the "syncytial ribbons" are found (1) in the roof of the amnion, (2) in the walls of the precocious coelomic cavity (see later), and (8) in the wall of the yolk-sac, as evidence that the differentiation of the ectoento-andmesodermhasnotyetaffectedseriouslythepotencies of the celsconcerned.

The Allantoic Cord

The condition of the allantoic representative in the H.R. 1 calls for detailedconsideration. Itisasolidcordofentodermwhich-due.allowance being made for the kink in the connecting stalk-arises in the median plane from the apex of a funnel-shaped diverticulum from the caudal wall of the yolk-sac. Itpassesatonceintotheconnectingstalkandmeasures0-185mm. from base to tip. At its distal end it takes part in the formation of the cloacal membraneoveranarea0*1mm. long.

Theearlyhistory of the allantois in the human embryoisstilveryobscure. In the Embryo Bi. 24 (Florian, 1933), which is at a slightly later stage of development than the Hugo embryo, an allantoic canal is present, measures 013mm. inlength,andtakesalargeshare in the formation of the cloacal membrane. Inalolderembryositispresentasapatent,tubulardiverticulum. It measures 019 mm. in the Manchester embryo (Hill & Florian, 1935), 014 mm. in the Thompson-Brash (1923), and 021 in the Peh. 1-Hochstetter(Rossenbeck,1923). Itsrelationshiptothecloacalmembrane in the se older embryos is variable. In the Manchester, the Dobbin (Hill & Florian, 1931) and the Thompson-Brash it takes no part in the formation of the membrane, whereas in the Peh. 1-Hochstetter and Grosser's embryos K1. 3 (1913) and Wa. 17 (1931) it forms a large part of it. In the Ingalls embryo (1918) there is a small area of ecto-entodermal connexion involving the allantois and separated by a gap from the cloacal membrane proper.

In the Embryo Hugo, according to Stieve's (1926) description, "ist kein deutlicherAllantoisgangodereineAllantoisbuchtvorhanden". A verysmall recess in the dorsal part of the caudal wall of the yolk-sac is tentatively regardedastheallantoicrepresentative. In embryos at an earlier stage of development than the H.R. 1 the condition of the allantoicrepresentativehasgivenrisetoconsiderablediscussion. Grosser (1913) identified an allantoic diverticulum in the Embryo Peters, and von Mollendorff originally described a large diverticulum of the yolk-sac in the Embryo O.P. as the allantois, but in both these embryos itisnow generally agreed that no allantoic representative is present. In the Embryo W.O., which is slightly older than the O.P., a solid mass of thickened entoderm projects dorsally from the roof of the yolk-sac at the caudal end of the em- bryonic area. VonMollendorif(1925)regardsthisastheprimordium of the allantois, and put forward the view that in the human embryo the allantois arises as a solid outgrowth which acquires a lumen later. Florian (1928) opposes this view and regards the entodermal outgrowth as the primordium of the cloacal membrane, which he has identified in the Embryos Fetzer and Bi.1. in the figuresalreadypublishedofBi.1(thefulldescriptionhasnot yet appeared) the allantois is represented by a solid entodermal outgrowth with a small central cavity, which projects from the caudal wall of the yolksacandfuseswiththeectodermoverasmallarea. Florian,however,isnow of opinion that this structure is not the definitive allantois but is destined to become incorporated in the yolk-sac and to be replaced by the true allantois atalaterstage. In the embryo Fetzernotraceofanyallantoicrepresentative ispresent.

The condition in the Strahl-Beneke embryo furnishes a close parallel to the condition in H.R. 1. The yolk-sac, which is slightly distorted, gives off an irregularly shaped recess at the junction of its roof with its caudal wall. This recess may in reality be due to the distortion, but it seems to represent the funnel-shaped caudal recess present in H.R. 1. From its summit a solid ento-dermal cord passes into the connecting stalk and comes into continuity near itsdistalendwiththeectoderm of the amnion. Itmeasures0*09mm.long. In the embryo T.F.theallantoisis,doubtfullyinmy opinion,representedby a small diverticulum from the dorsal end of the caudal wall of the yolk-sac, sothattheconditionisnotdissimilartotheconditionof the embryo Hugo. In the Embryo Meyer a funnel-shaped diverticulum projects dorsally from the caudal end of the yolk-sac roof into the connecting stalk. The end of the diverticulum is formed by a short, solid mass of entoderm, which may possibly bemoreextensivethanisrepresentedinMeyer'sfigures. Florian(1929)isof opinion that a cloacal membrane is probably present in connexion with the allantoicoutgrowth,whichmeasuresnotlessthan0-12mm. inlength. In Grosser's Embryo H.Schm. 10 (1981) the allantois isrepresented by a tubular diverticulum with relatively thick walls and a very narrow lumen. It is 005mm. longand in the neighbourhoodofitssolidtipitsepitheliumisfused withtheamnioticcovering of the connectingstalk.

From this evidence it is possible to draw some tentative conclusions with reference to the early development of the allantois. The embryos W.O., Bi. 1, Strahl-Beneke, H.R. 1, Meyer and H.Schm. 10 form a series which. suggestthattheprimordium of the allantoisisasolidentodermalcordwhich establishes continuity with the ectoderm of the amnion covering the connectingstalkatanearlystage. Itacquiresitslumenlater,probablyasan extension of the yolk-sac cavity into the cord. On the other hand, the conditions found in the Embryo Hugo and the Embryo T.F. make itclear that the actual time of appearance of the allantoic representative is subject to considerablevariation. Ifthesmalldiverticulafromtheyolk-sacinbothembryos are accepted as representing the allantois, only two alternative explanations arepossible. (1)Theallantoicrepresentativeappearsasasolidcordwhich soon becomes atrophied and disappears, its proximal end forming the site of the hollow allantoic canal. (2) The allantois may develop either as a solid entodermal rod, or as a small, hollow diverticulum which rapidly enlarges and grows into the connecting stalk. On the whole it seems more probable that the development of the allantoic primordium has been delayed in both cases, and that the identification of the small diverticula as allantoic representatives is not justified. The question, however, is complicated by the relationships exhibited by the cloacal membrane.

The Cloacal Membrane

This remarkable structure has been very fully investigated by Florian, who has succeeded in identifying it in embryos as early as the Fetzer and the Bi. 1. Further he believes that it is probably present in the Embryo W.O. It was therefore confidently anticipated that a cloacal membrane would be present in the H.R. 1, but no connexion could be observed between the roof of the yolk-sacandtheshieldectoderm. Prof.Florian,towhomIhadsent photographs of the sections, identified it caudal to the shield, and further examination of the sectionsshowedthathisidentificationwasjustified. In the H.R. 1 (Text-figs. 19 and 21) the cloacal membrane is associated with the terminal0-1mm. of the allantoiccordandextendsbetweenitandtheamniotic covering of the connectingstalkimmediatelycaudaltotheshield. Itiscut obliquelybutitsidentificationisunequivocalinsections19-7and19-8,though it is not quite so convincing in sections 19-6, 19 9 and 19-10. The ectodermal cels of the mostcaudalpart of the membrane (upperpart in the figures)were mistaken at first for a large blood island, but further examination demonstrated theircontinuity with the cels of the allantoic cord and with the cells lying cranial to them (below, in the figures). These celscontain a few of the darkly staining droplets which have been mentioned in connexion with the shieldectoderm. Itisimportanttoobservethat in the Strahl-Benekeembryo also the ectoderm concerned in the formation of the cloacal membrane is amniotic. On the other hand, both in the Bi.1 and the Fetzer embryos Florian has identified the membrane within the limits of the shield and in Embryo Bi. 24 it is situated partly in the shield and partly in the connecting stalk. Inolderembryositissituatedeitherwhollywith in the shield(Thompson-Brash, Manchester, Dobbin, etc.) or it may involve both the caudal end of the shield and the adjoining amniotic ectoderm of the connecting stalk (Peh. 1-Hochstetter, Sternberg (1927), Grosser's K1. 3, etc.).

The question at once arises as to whether the cloacal membrane in H.R. 1 and in the Strahl-Beneke embryo is identical with the structure described by Florian in the Fetzerembryo(1930)and in the Hugoembryo. InbothH.R.1 and the Strahl-Beneke embryo the cloacal membrane is outside the shield areaandinvolvestheallantoiccordonly. in the Fetzerembryoitlieswithin the limits of the shield and involves the roof of the yolk-sac (the allantois is notyetpresent). Itmaybe,asWyburn(1937)appearstobelieve,thatthe cloacalmembrane in the Fetzerembryoindicatesthesiteatwhichtheallantois will subsequently develop, but Florian in his Figs. la and lb places itdefinitely with in the area of the shield. Ifthetwostructuresareidenticalitfollowsthat, in order to account for the conditions found in the older embryos, the cloacal membranemay appeareither in the connectingstalkandlaterextendcranially on to the shield, or in the shield and later extend caudally on to the stalk. That such a variability in the origin and growth of the cloacal membrane should occur is extremely improbable, and one is forced to the conclusion that ifFlorian'sidentification of the membrane in the embryo Fetzerisjustified, thetwostructurescannotbeidentical. Itwouldthenbenecessary,inorderto explain the ecto-entodermal fusion in the Strahl-Beneke and the H.R. 1, to falbackon thepossibleoccurrenceofa "canalisamnio-allantoicleus".This structure was described by Schauinsland (1902) in Lacerta and was called in by Ingalls to explain a very small area of amnio-allantoic fusion (1918). Schauinsland's description for Lacerta is far from convincing and there is no evidence available to justify the suggestion that the rudiments of such a structure are normally present in the human embryo, and the fact that amnioallantoiefusionispresent in the Strahl-Beneke,theH.R. 1and theH.Schm. 10 embryosissufficienttoshowthatthefusionisanormalfeature. Indiscussing Grosser's H.Schm. 10, Florian (1933) makes it clear that he regards ectoentodermal fusion and not contact as the criterion for the recognition of the cloacalmembrane. Herejectsanareaofcontactatthecaudalend of the primitive streak within the shield area and accepts the amnio-allantoic fusion in the connectingstalkasthecloacalmembrane. in this wayhebringsthe H.Schm. 10 into alignment with the Strahl-Beneke and the H.R. 1.

The whole question, however, bristles with difficulties and can only be solvedwhenfurthermaterialhasbecomeavailable. Meantime,Iinclineto the view that the cloacal membrane develops later than the allantois and in close association with it, and at a later period extends into the caudal end of the shield. This is a modification of the view recently put forward by Wyburn (1937) who, however, had access to no material younger than the Embryo MacIntyre 1, which has well-developed headand tail-folds and measures 1*4mm.

The Prochordal Plate

The patch of thickened entoderm which forms such a conspicuous feature in the roof of the yolk-sacinsections199-20'3hasalreadybeenidentifiedas theprimordium of the prochordalplate. Itmeasures0075mm. longx005mm. wideanditscraniallimitlies0-12mm. fromthecranialborder of the shield. Except towards its right margin it is connected to the basement membrane of the shield ectoderm over an area which varies in extent from section to section. Itiscertainthattheconnexionwasbothrealandextensive,probably considerably greater than is represented in the median section of the model (Text-fig.2). Inmost of the sectionsin which it iscuttheplate consists of from ten to twelve large, vesicular cels, with large, round or oval nuclei and with only suggestions of cel boundaries. The cytoplasm is lightly staining and contains many vacuoles. The ventral surface of the plate is covered with the fine coagulum content of the yolk-sac, and it is uncertain whether or not the plate is continuous at its caudal end with the cranial end of the head process,althoughthecontinuity,ifitexists,isexceedinglytenuous.

Florian (1933) has described and figured a patch of similar cells in the roof of the yolk-sac in the Embryo Fetzer, but in this case the cells form a single layerandlieimmediatelycranialtotheprimitivestreak. A corresponding patch of thickened entoderm is present in the Strahl-Beneke embryo and its position and the characters of its constituent cells indicate quite clearly that we are dealing here with the same structure as is present in H.R. 1. In the Embryo Hugo, on the other hand, Stieve (1926) makes no mention of the existence of any corresponding structure although his Figs. 12 and 13 are rather suggestive, and in the Embryo H.Schm. 10 Grosser (1931 b) says " Eine Prachordalplatte lasst sich nicht mit Sicherheit nachweisen ".

in the Bi.24andolderembryostheprochordalplateisaconstantstructure.

The prochordal plate in the Strahl-Beneke embryo and in H.R. 1 might conceivably have another interpretation. The vesicular character of the cells concerned might be regarded as a degenerative change prior to desquamation toprovidespace for the growingheadprocess. Ontheotherhand,thenuclei are healthy and active in appearance, and desquamation does not necessitate apriorthickening. Attention should be drawn to the adhesion of the plate to the basement membrane of the shield ectoderm, and to the relation existing between the plate and the precocious coelomic cavity (see later). Both point to the possibility of a relationship between the prochordal plate and the buccopharyngealmembrane.

Contents

The yolk-sac contains in places a small amount of very lightly staining and foam-like coagulum which, although situated centrally for the most part, isalsopresentinmany sectionsincontactwiththeroofandthecaudalwall. ItisseeninPI.3,fig.1incloserelationtotheprochordalplate.Thepresence of such a coagulum in the yolk-sac has been noted by many observers and may indicate secretary activity of the entoderm, or it may be the result of post-mortemchanges. Inadditiontheyolk-saccontainsafewdesquamated entodermal cels, especially in the region of the head process as already stated.

Entoderm Cysts

A careful search was made for isolated entodermal cysts in the chorionic cavity, although the fact that the specimen had been reblocked after the cavity had been opened implied that a negative result would not necessarily mean that none had been present originally. In 7-2-9-2 a patch of undoubted entodermal cels was discovered which hadbecomeentangledinafilmofcoagulum. Itcomprisesfourtotencelsin each section. The nuclei are rounded or oval, lightly staining but well fixed. Althoughthecellsareinmany instancesvesicularinappearance,theyforma solid clump with no sign of cavity formation.

The Intra-Embryonic Mesoderm

Except in the region of the median plane caudal to the prochordal plate, the intra-embryonic mesoderm forms a continuous open meshwork over the wholeof the embryo nicarea. Roundtheperiphery of the shieldthecelsare not very numerous but they are connected to one another by branching processes. Wherevertheyreachtheperiphery of the shieldtheybecomecontinuous with the primary mesoderm and at the caudal end they pass into the connectingstalk. Inplacesthemesodermcelsfilupthewholeinterval between the ectoderm and the entoderm, but as a general rule shrinkage spaces intervene between the mesoderm and one or both of the two other layers. Inmanysituationsthemesodermcellsappeartobespringingdirectly from the deep surface of the shield ectoderm and elsewhere they are often connected to it by fine cytodesmata; in other situations they are intimately related to and apparently continuous with the entoderm of the roof of the yolk-sac. There is no evidence in H.R. 1 to show whether these cells are actually derived from the primitive streak, the primary mesoderm, the shield ectodermortheentoderm. Stieve(1926)believesthattheintra-embryonic mesodermatthisstageisderived(1)fromtheprimarymesoderm,(2)fromthe primitive streak and (3) from the entoderm of the roof of the yolk sac. The histological appearances, however, may be very misleading and I am of opinion that the Embryo H.R. 1 provides no definite evidence in this connexion.

Typical mesoderm cels can be seen in Text-figs. 23 and 24 apparently arising from the caudal end of Hensen's node, and others are seen in Textfigs. 21 and 22, apparently arising from-the cranial end of the node and lying dorsaltotheleftedge of the headprocess.

To the right of the median plane the sections become more oblique owing tothecurvature of the shield.Two consecutivesectionspassbetweentheright wall of the yolk-sac and the shield ectoderm (Text-fig. 30). In these the fusiform cels of the intra-embryonic mesoderm form a continuous openmeshed network from the cranial to the caudal periphery of the embryonic area by means of their interconnected branching processes.

In sections 18-9-19-3 the exocoelom encroaches on the embryonic area at the cranial end of the shield and the primary mesoderm which lines the invagination is directly continuous with the intra-embryonic mesoderm, so that it is impossible to be certain where the one ends and the other begins. In the two succeeding sections the intra-embryonic mesoderm at the cranial end of the shield contains a cavity which appears to be shut off from the exocoelom,whichnolongerencroacheson the embryo nicarea. in the nine succeeding sections the same cavity is present (P1. 1, fig. 2), although it is partly interrupted in 19-8 and is damaged in 19-11, and it extends cranially practicallytothelimit of the shield.Thiscavity-containingmesodermextends caudallytotheregion of the prochordalplate. Only two possible interpretations can be suggested for this cavity, for the appearances exclude the possibility of vascular formation. Eitheritisan artefactduetoshrinkage,oritisaprecociouslydevelopingcoelomicspace. At firstsighttheformerexplanationisthemoreprobable. Notraceofanysuch cavityispresentinotherembryosatthisstage. In the Dobbin embryo (Hill& Florian,1931),whichis0'96mm.long,theminutecavitiesapparent in two of the sections are regarded by the authors as very doubtful coelomic formations. OntheotherhandtheappearancesinH.R.1arestronglysuggestive of coelomic formation. The continuity of the cavity through so many sections alone throws grave doubt on the likelihood that we are here dealing withanartefact. Insome of the sectionscytodesmataconnectthedorsalwall totheshieldectoderm,butnocytodesmataarefoundwith in the cavity. In two of the sections(19.12and20.1)thewalls of the cavitypresentthe"syncytial ribbon" appearance which has already been shown to be indicative of goodfixationandpreservation. Oneisthereforeforcedtoconcludethatthe cavity is a natural space within the intra-embryonic mesenchyme and as such it can only represent a precociously developed coelomic space.

The Primary Mesoderm

On theamniontheprimarymesodermformsasinglelayerofcels,except around the periphery of the embryonic shield where it forms a rather thicker layerontheupturnedrim.Towardsthecranialandcaudallimits of the amniotic roof no difficulty is experienced in distinguishing the mesodermal from the ectodermal layer. The mesodermal cels are fusiform with prominent nuclei: the ectodermal cels also are elongated but their nuclei do not project above thegenerallevel. Overthecentralpart of the ro of the twolayersareindistinguishable and in the sections they form a ribbon-like syncytium (Text- fig. 23). The same appearance has already been noted in the wall of the coelomicspaceand in the wall of the yolk-sac. in the lattersituationand in the amniotic roof similar areas are present in both the Strahl-Beneke and the Hugo embryos. The condition has been discussed at an earlier stage.

On the yolk-sac two types of cells can be identified without difficulty. The firsttype,whichgreatlyexceedsthesecondinnumber, comprisescelswhich are almost epithelioid. Their nuclei are rounded and relatively large; they do not stain darkly but their nucleoli are conspicuous and they contain a fine chromatinnetwork. Insomeplacesthecelsofthistypeformasinglelayer, but in many others they form clumps of variable size, containing from five to thirtycels.The second type comprises a number ofelongated, flattenedcels. They are found in places covering the entoderm but they are more commonly situatedamongsttheclumpsofcels of the firsttype,wheretheyhelptoform thewallsofintercellularspaces.

These clumps are regarded as blood islands and the occasional spaces as earlystages in the formationofvascularspaces. in the islandsthecelboundaries are indistinct and the cytoplasm is pinkish in tint. The vascular spaces are neither so large nor so numerous as those found in the same situation in the Embryo Hugo; they are al small in size and they do not communicate with one another.

The connecting stalk extends from the apical part of the chorionic vesicle to the caudal end of the embryonic area and the adjoining part of the roof of theamnion. Attentionhasalreadybeendrawntothegapwhichisapparent between the connecting stalk and the dorsal surface of the funnel-shaped diverticulumfromthecaudalsurface of the yolk-sac. Although,asstated, the gap is regarded as a tear in the embryonic attachment of the connecting stalk, the appearances of its boundaries would lead one to suppose that it is a natural interval, and the evidence in favour of the interpretation put forwardismainlyofanegativecharacter. Ifitisnotatear,thentheconnecting stalk has a very narrow attachment to the embryo and a very wide attachment to the amnion, a condition which is not impossible but which is exceedingly improbable. The allantoic cord has a pronounced bend and its connexion with the yolk-sac has been displaced considerably to the left of the medianplane. Suchaconditioncanbeexplainedsatisfactorilybyatearof theconnectingstalk,asalreadyindicated. Finally,nocorrespondinggapis presentinotherearlyhuman embryos.

The ab-amniotic surface of the stalk is covered with loosely arranged and loosely connected epithelioid cells which occasionally form small surface clumpsverysimilartothebloodislandsontheyolk-sac. Asthestalkistraced towards its chorionic attachment it becomes less cellular and is looser in texture. Actualspaces-whicharerare in the vicinityof the embryo apart from contraction spaces-are more numerous and are relatively large, much larger than the spaces in the wall of the yolk-sac. Many of them are surrounded by flattened cells which indicate their vascular character (Text- fig.34).

As already stated,theconnectingstalkcontainstheallantoiccordandthe whole extent of the cloacal membrane.

The Age Of The Embryo

The temptation to regard the dimensions of the embryonic shield as a reliable guide to the fertilization age of the embryo has disappeared since Stieve(1926)publishedhisdescriptionof the embryo Hugo. Inthatcasea period of only 131days elapsed between insemination and operation. If only 12 hr. is allowed for the interval between insemination and fertilization, the fertilization age of the Embryo Hugo would be 13 days. In Bryce's Embryo T.B. 1 (1908) the history was not less reliable than in the Embryo Hugo. Anintervalof161dayselapsedbetweeninseminationandtheabortion. Bryce allowed 24 hr. for the interval between insemination and fertilization and a period of 24-36 hr. between the death of the ovum and the actual abortion, and finally concluded that the fertilization age of the embryo was 13-14 days. Thus 13 days may be regarded as the minimum for T.B. 1 and as themaximumfor the embryo Hugo. ButthedevelopmentalstageofT.B.1 is very considerably earlier than that of the Hugo, and according to Bryce's reckoning the latter would take its place in his series between the Leopold and the Reichert at an estimated age of 17-18 days.

Consideration of these two embryos makes it abundantly clear that the rate of development isby no means constant for human embryos and that two specimens of the same fertilization age may differ in developmental stage by asmuchas3or4days. SuchadisparityhadbeenforeshadowedbyRabl (1915), who showed that at 7 days 8 hr. fertilization age, rabbit embryos of the same litter might differ in shield length by as much as 50 %.

In the Embryo H.R. 1 unfortunately no evidence is available as to the actualfertilizationage,butthedevelopmentalstageisclearlyearlierthanthat of the Embryo Hugo and as clearly later than that of the Embryo Strahl- Beneke or the Embryo T.F. The operation of hysterectomy was performed on the day before the next period was expected to begin, or 27 days after the commencement of the last period. As the normal duration of the periods was 6-7days,themaximum fertilizationagewouldbe19days,if24hoursare allowed as the interval between insemination and fertilization. Assuming a developmental rate as rapid as that of the Embryo Hugo, the minimum fertilization age would be in the neighbourhood of 12j days. The actual fertilization age lies somewhere between these two extremes and is probably about 151days. Is the Embryo H.R. 1 a normal embryo? This question must arise and deserves careful consideration in connexion with every young human embryo, anditisoftenexceedinglydifficulttoanswer. Evidenceisgraduallyaccumulating to show that even the median plane structures are by no means constant in their relative times of appearance. This variability has been stressed by Grosser (1931b,c) in two recent papers, and must be borne in mind when coming to a decision with regard to the Embryo H.R. 1.

Our knowledge of the earlystages of the human embryo isbasedonmaterial obtainedfromoneofthreesources,viz.:(a)fromoperations,(b)fromabortions, and(c)frompost-mortemexaminations. Ingroups(a)and(b)thecondition of the endometrium must always be suspect, for, presumably, the specimens would not otherwise have been obtained. However, the condition of the endometriumingroup(a)canalwaysbeexamined in the neighbourhood of the ovum, and in H.R. 1 it displayed no obviously pathological features. The specimens falling into group (c) may or may not be free from suspicion as regardsthecondition of the endometrium,buttheynearlyalwaysgiveriseto doubts and difficulties because of their imperfect preservation due to the interval between the occurrence of death and the performance of the autopsy.

In H.R. 1 the pronounced curvature of the shield, which has influenced the size of the yolk-sac, must be regarded as the result of an abnormal growth process. Itmayhavebeenexaggeratedduringfixation,butprobablynotto anygreatextent. Doesthisabnormalityofshapeimplythatnoimportance attaches to any of the features which this embryo displays? Almost certainly not, for careful comparison with other embryos has shown that the median plane structures, with the exception of the coelomic cavity, are in a stage of developmentwhichareinharmonywiththeknownfactsandwithoneanother. The primitive streak and its two nodes, the head process, the prochordal plateandthecloacalmembranecanalberegardedasnormal,withthepossible exception of the head process. The presence of a coelomic cavity must be regarded as evidence of precocity only, for it conforms in every way to the conditionswhichourknowledgeofitsnormaldevelopmentwouldhaveledus toanticipate.

To sum up, the Embryo H.R. 1 is admittedly abnormal in shape and is precocioussofarascoelomicdevelopmentisconcerned,butinotherrespects, with the possible exception of the head process, itisnormal; and the evidence which it suppliesinconnexionwiththedevelopment of the cloacalmembrane, the prochordal plate and the allantois is entitled to its fair share of weight.

Summary

1. The Embryo H.R. 1isatapproximatelythesame stageofdevelopment asGrosser'sH.Schm.10. ItisatalaterstagethantheStrahl-Benekeandthe T.F. embryos, and at an earlier stage than the Embryo Hugo. 2. The curvature of the shield is the result of a growth process, although itmayhavebeenslightlyexaggeratedpost-mortem. Itisregardedasan indication of abnormal growth due, in part at least, to the shape of the chorioniccavity. 3. Theprimordium of the headprocessispresentandexhibitsanearly stageofdevelopment. It consists of agroupofcolumnarcellswhichreplace the entodermal roof of the yolk-sac over a limited area. 4. The allantois is represented by a solid entodermal cord, which is regardedasthenormalcondition of the firststageofthisstructure. 5. The primordium of the prochordal plate is present. 6. The cloacal membrane is situated beyond the caudal end of the shield. It is suggested that the membrane normally develops first in this situation and then extends cranially to involve the shield. 7. The primitive streak does not extend to the caudal limit of the shield. An earlystage in the developmentofHensen'snodeisdescribed. 8. A precociouslydevelopedcoelomicspaceispresent in the cranialpart of the shield. The appearances suggest that the space communicates caudally withtheexocoelom.

Ihavetoacknowledgegratefullymy indebtednesstotheUniversityof Edinburghforgrantingme permissiontopublishthiscommunication,which, with slight modifications, was submitted in 1937 as part of a Thesis for the degree of M.D. I am also indebted to Mr G. A. Walker, senior technical assistant in the Anatomy Department, Guy's Hospital Medical School, who was responsible for the micro-photographs, and to Miss P. M. Lariviere, who executedthelinedrawings. Ialsoacknowledgewithgratitudethefactthat the expenses of illustration have been defrayed in part by a generous grant fromtheClinicalResearchCommitteeofGuy'sHospitalMedicalSchool.

NOTE The sections were submitted to Prof. Florian, who had seen the photographs and had indicated his interest in the specimen. Early last year he sentme agraphicreconstructionofamediansection-madeundercircumstancesofquiteextraordinarydificulty. Iam verygreatlyindebtedtohim for the timeandtroublehehasdevotedtothisembryoand for the interesthe has taken in it. Unfortunately, for obvious reasons, I have not had an opportunityofdiscussingwithhim thepointson whichourreconstructionsdifer, and I have therefore not included reference to his reconstruction in the generalargument. Ifel,however,thatitisonlyfairtoreadersofthispaper to include the figure (Text-fig. 35) which Prof. Florian has given me permission to publish, and the measurements made from it are given in Table 1. Itmustbeemphasizedthatthecurvatureof the embryo andtheobliquityof the sectional plane make accurate reconstruction difficult by any method, and thesedificultiesareincreasedby thepartialtear in the connectingstalkand the consequent kinking at the caudal end of the embryo (p. 6). It is not surprising, therefore, that Prof. Florian and I are at variance as to the inclination of the sectional plane, and this accounts for the difference in our estimates of the length of the shield.

Text-fig. 35. Graphic reconstruction of a median section through H.R. 1 made by Prof. J. Florian. x150. Solidblack=undifferentiatedectoderm: horizontallines=ectoderm. P.8.= Primitive streak; Cl.m.= Cloacal membrane; S.p.=Sectional plane; H.p.=Plane of Prof.Florian'sdorsalprojection(notfigured).

Further, what I have identified as Hensen's node, Prof. Florian regards as the primitive streak at an early stage of differentiation, and what I have identified as the primitive streak, he regards as undifferentiated shield ectoderm. Lastly,Prof.Floriandoesnotrecognizeeitheraheadprocessora prochordalplateinH.R. 1. Readerswillbeabletoformtheirown opinionsby examination of the figures.

The other differences between the two reconstructions are of minor importance.

On the other hand, the two reconstructions are in agreement as to the general shape and outline of the shield, the allantoic primordium and the position of the cloacalmembrane.

References

BENEKE, R. & FromwA, J. (1931). Anat. Anz. Erginzungsheft zum 71.

BRYCE,T.H.(1908).EarlyDevelopmentandImbedding of the HumanOvum. Glasgow.

- (1924).Trans.Roy.Soc.Edinb.53,pt.m.

COREDG, H. K. (1925). Lehrbuch der Entwickelungegechichte de8 Menechen. 2. Aufi. Munchen: Bergman.

FALKINER, N. M. (1932). J. Obat. Gynaec. 39. FETZM,M.&SoH WN,J.(1929).Anat.Anz.67.

-~-~(1930)Z.milr.-anat.Forach.21.

FLORIAN, J. (1927). Anat. Anz. Ergiinzungsheft zum 63. (1928). Z.mikr.-anat.For8ch.13.

(1933). J.Anat.,Lond.,67.

FLORIAN, J. & VOLKER, 0. (1929). Z. mikr.-anat. Forsch. l6M GROSSER, 0. (1913). Anat. Hefte, 47.

(1931 a). Z. ges. Anat. 1. Z. Anat. EntwGesch. 94. (1931b). Anat. Anz. Erganzungsheft zum 71. (1931c). Anat. Anz. Erganzungsheft zum 72.

HAMILTON,W.J.(1937). Tranm.Roy.Soc.Edinb.79,pt.I. HEUSER,C.H.(1932). Contr.Embryol.CarnegieIn8t.21.

HILL, J. P. & FLORIAN, J. (1931). Philo8. Trans. B, 219.

- (1935). J.Anat.,Lond.,69.

INGALLS, N. W. (1918). Contr. Embryol. Carnegie Inst. no. 23, 7. MACINTYRE,D.(1926). Trans.Roy.Soc.Edinb.55,pt.I.

MEYER, P. (1924). Arch. Gyndk. 122.

V. MULLENDORFF, W. (1921a). Z. gea. Anat. 1. Z. Anat. EntwGesch. 62.

(1921b). Z.ge.Anat.1.Z.Anat.EntwGmech.62.

(1925). Z.ges.Anat.1.Z.Anat.EntwGesch.76.

ODGERS,P.N.B.(1936). J.Anat.,Lond.,71.

RABL,C.(1915). Arch.mikr.-anat.Forsch.88.

RAMSEY,E.M.(1937). Contr.Embryol.CarnegieInst.26.

ROSSENBECK, H. (1923). Z. ges. Anat. 1. Z. Anat. EntwGesch. 68. SCHAUINSLAND,H.(1902). ArticleinHertwig'sHandbuch,1.

SCHLAGENHAUFER & VEROCAY (1916). Arch. Gynak. 105.

v. SPEE, GRAF (1896). Arch. Anat. und Physiol., Lpz. STERNBERG, H. (1927). Z. ges. Anat. 1. Z. Anat. EntwGesch. 82.

STIEVE,H.(1926). Morph.Jb.7. (1936). Jb. Morph. Mikr. Anat. 40.

STRAHL, H. & BENEKE, R. (1910). Ein junger menschlicher Embryo. Wiesbaden. STREETER,G.L.(1936). Anat.Rec.67. (Abstract.)

TEACHER,J.H.(1924). J.Obst.Gynaec.31.

THOMPSON, P. & BRASH, J. C. (1923). J. Anat., Lond., 58.

WYBITRN, G. M. (1937). J. Anat., Lond., 71.

Explanation Of Plates

PLATE 1

Fig. 1. The ovum in situ, showing the triangular shape of the chorionic cavity, the position of the embryo with in the chorion,the vili and intervillous space and the large blood clot which obscures the point of entry. The dilated uterine glands in the stratum spongiosum are con- spicuous and attention is drawn to the difference between those in the immediate vicinity of the ovumandthosefartherdistant. x16.

Fig. 2. Part of section 19-6 showing the cranial end of the shield and the (?) coelomic space. x634.

PLATE 2

Fig. 1. Part of section 19*11 showing Hensen's node and the head process. (Cranial end of shield toleftandcaudalendtoright.) x950. Fig.2.Thecorrespondingpartofsection19-12. x950.

PLATE 3

Fig.1. Partofsection19-12showingthepatchofthickenedentodermidentifiedastheprochordal plate. Note itsconnexionwiththeshieldectoderm. x950.

Fig.2.Thecorrespondingpartofsection20-1. x950.