Paper - A Young Human Embryo (Embryo Dobbin) with Head-Process and Prochordal Plate

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Hill, J. P. and Florian, J., 1931. A Young Human Embryo (Embryo Dobbin) with Head-Process and Prochordal Plate. Phil. Tran. Roy. Soc. London B, 219, 443-486.

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By J. P. HILL, D.Sc., F.R.S., and J. FLORIAN, M .D.

(From the Department of Anatomy and Embryology, University College, London.)

(Received July 27, 1931.- Read November 19, 1931.)

(Plates 29-35)


Our knowledge of the early development of Man has made marked progress during recent years, numerous fairly Well—preserved embryos having been described in greater or less detail. There are still, however, numerous gaps in our knowledge, pertaining not merely to points of detail but to fundamental questions, so that any early embryo reasonably Well preserved and reasonably Well sectioned is deserving of careful study.

Our specimen belongs to the comparatively late presomite stage represented in the literature by such embryos as K113 (GROSSER, 1913), Wa17 (GROSSER, 1931), Peh.1- Hochstetter (ROSSENBECK, 1923), and the embryo of INGALLS (1918), and characterised by the presence of an elongated primitive streak, a luminated chorda-process and a prochordal plate. We offer no apology, however, for presenting a fairly detailed, description of yet another example of this stage, since our embryo amongst other things provides a more complete picture of the cranial region of the head~process than any specimen hitherto described.

History and Treatment of the Specimen

The chorionic vesicle, the embryo of which forms the subject-matter of this paper, was presented to one of us (HILL) by Dr. ROY DOBBIN, of Cairo, through the kind offices of Professor D. E. DERBY. In appreciation of his valuable gift, We have much pleasure in associating Dr. DOBBIN’s name with the embryo.

The clinical history supplied by Dr. DOBBIN is as follows: “ Coitus, 6.10.23 ; effort probably causing abortion, 21.10.23; first bleeding, 22.10.23; abortion (painless), 23.10.23."

Although an abortion, we see no reason to regard the specimen as other than perfectly normal. The chorionic vesicle (which was preserved in spirit) was, when received, somewhat flattened and shrunken (fig. 1, Plate 29). Except over a small area on one side (approximately 3 X 2 mm. in diameter), which was almost bare, the vesicle possessed a fairly uniform covering of short, close~set, branched villi (fig. 2, Plate 29), to which at one point a small fragment of blood—clot adhered. Including the villi, its dimensions in alcohol were as follows : 11 - 5 mm. (in long diameter) X 8 -5 mm. (in short diameter) X 45 mm. (in thickness). After clearing in oil of cedar-wood, the corresponding internal diameters were 9 mm. X 5-5 mm. X 2-5 mm. 2 The vesicle, after being photographed and drawn, was dehydrated and cleared in oil of cedar-wood. A small portion of the chorion, including the bare area, was then carefully removed, and through the opening so made it was possible, fortunately enough, to locate the embryo under the binocular dissecting microscope. The embryo was then isolated along with the segment of the chorion to which it was attached, and stereo-photographs were successfully taken of it, in the cleared condition in oil of cedar—wood. Subsequently Mr. A. K. MAXWELL, with the aid of these photographs and the camera lucida, made the beautiful drawings representing the left lateral and dorsal aspects of the embryo which are reproduced as figs. 3 and 4, Plate 29. These figures are, we believe, unique, in that they are the only illustrations extant of the dorsal andlateral views of the human embryo at this particular phase of development which have been made directly from the embryo itself, and not from models or reconstructions.

The embryo and the related piece of chorion were double—embedded in cedar-wood oil-pyroxelene and paraffin, and cut cranio—caudally by H. BARKER into a complete and really very fine series of sections at 8 (1., the sectional plane being almost exactly transverse to the long axis of the embryo. The state of preservation of the latter proved to be by no means perfect cytologically, but is sufficiently good to ustify us in giving a fairly detailed account of its structural condition. The most obvious fixation—defect is the partial disintegration of the most cranial portion of the Shield-ectoderm, the granular detritus resulting therefrom lying partly in the amniotic cavity and partly below the ectoderm, in a space enclosed between the latter and the detached basement membrane. The fact that the cranial region of the early embryo is the first part to undergo dissolution seems to be Well recognised. Inspection of fig. 3, Plate 29, will show that the embryo no longer occupies its normal position in relation to the chorion, but has been displaced in the ventral direction. Fortunately the deformation accom- panying this displacement has affected only the most caudal part of the embryo and the connecting stalk, and is not of a serious character.

The following measurements were made whilst the embryo was still in oil of cedar- Wood, but must be regarded as approximate only. The lettering refers to text—fig. 2 :-

Anterior margin, embryonal shield to region of cloacal membrane (A-Cl.) 0.98 mm
Vertical diameter (D-F) of yolk-sac 1.092 mm
Antero-posterior diameter of yolk-sac, near its mid-region 0.98 mm
Vertical height, amnio-embryonal vesicle (D-E) 0.468 mm
Length of yolk-sac process 0.88 mm

The measurements of the embryo based on the sections and graphic reconstructions are set forth, along with those of other early embryos, in the tables provided at the end of this paper (p. 480-81).

We take this opportunity of expressing our very grateful thanks to Dr. ROY DOBBIN and to Professor D. E. DERBY for the gift of this interesting embryo. We are further greatly indebted to Professor J. S. B. STOPFORD for the loan of the presomite Manchester embryo, No. 1285 ; to Professor J. C. BRASH for the opportunity of examining the sections of the Thompson-Brash embryo; to Hofrat Professor F. HOCHSTETTER for permission to study the Peh.,-Hochstetter embryo, described by ROSSENBECK, 1923 ; and to Dr. O. BITTMANN, Brno, for the gift to one of us (FLORIAN) of the Bi(ttmann) 24 embryo, to which reference is made on pp. 466—7 . We desire also to thank Mr. A. K. MAXWELL, Artist to the Department, for the care and skill he has expended on the illustrations on Plates 29-34, Which, with the exception of figs. 1 and 2, are all based on photomicrographs; and to Mr. F. J. PITTOCK for his invaluable assistance in photography.

The Embryo as a Whole

The embryonal body, represented by the two vesicles——the amnio-embryonal and yolk—sac vesicles-——-is attached by a very distinct connecting stalk to the chorion (fig. 3, Plate 29). It will be observed that, as has just been mentioned, the dorsal surface of the amnio~embryonal vesicle does not face the chorion, as is the rule in this stage of development, the embryonal shield forming an angle of about 125° with the inner surface of that membrane. This is evidently due to the artificial displacement of the embryonic body, including the connecting stalk, in the ventral direction. At the same time the embryo and the connecting stalk have sufiered a slight rotation round the long axis of the body towards the left side (as may be seen from fig. 4, Plate 29). As is usual at this stage, the yolk—sac vesicle is distinctly larger than the amnio-embryonal.

Text-Fig. 1. Graphic reconstruction of the dorsal view of the embryonic formation; Dobbin embryo X 75. - Outlines of the embryonal shield (E.s.), yolk-sac (Y.s.) and connecting stalk ( indicated by a thick line-; outlines of the allantoic canal (ALI = tubular part, AL2 =2: vesicular part) indicated by interrupted lines; outline of the caudal end of the amniotic cavity (Amn.) dotted; cloacal membrane (UL) lined longitudinally where indistinct, cross—hatched where distinct ; P7‘..S‘. =2 primitive streak; chorda process and chorda canal lined transversely; lateral mesodermal bands lined transversely and dotted; ventral openings of the chorda canal marked by arrows on the right, its dorsal opening by an arrow on the left. ++ = HENSEN’s knot ; prochordal plate marked by large dots. On the scale is indicated the level of every fifth section. The median plane is indicated by an arrow.

Text-Fig. 2. Graphic reconstruction of the median section through the Dobbin embryo X 7 5. - Ectoderm black; endoderm lined horizontally; primitive streak lined vertically; +=HENSEN’s knot; chorda-process lined obliquely; prochordal plate ( lined horizontally and dotted; mesoderm dotted; cloacal membrane (O'l.) cross—hatched. Limits of the embryonal shield marked by arrows. AL; 2: proximal (tubular) part of the allantoic canal; Al.2 == its distal (vesicular) part. Hor. —-—=. horizontal plane (the plane of the projection in text-figs. 1 and 8). The caudal part of the embryonic formation, including the cloacal membrane, the allantoic canal, the caudal part of the d amniotic cavity and the connecting stalk are projected into the median plane, and their outlines are marked by interrupted lines. , The cell-layer underneath the intermediate region and the most cranial portion of the head-process, the significance of Which is doubtful, is left White.

The embryonal shield forms in the lateral view (text-fig. 2) an open S-shaped curve. It reaches cranially up to the contour-line formed by the amniotic and yolk—sac vesicles, and ends caudally just in front of the connecting stalk.

The sections show that the embryonal shield over its cranial two-thirds is not flat as is normal at this stage, but is dorsally concave so that in the lateral view of the embryo (fig. 3, Plate 29) the ventral surface of the left half of the embryonal shield is turned towards the observer and appears much darker than the right half, which is only partly visible.

The yolk—sac is roughly conical in form; its broad base underlies the embryonal shield, whilst its apical region narrows to form a spike-like projection. This latter is prolonged into a filamentous process (about 0 - 88 mm. in length) which exhibits three enlargements, an elongated thickening close to the apex of the yolk-sac, an oval swelling near the middle of its length and a quite minute nodule close to its termination (fig. 3, Plate 29). It ended freely, without reaching the chorion. The entire process represents the yolk~sac process first described by STRAHL and BENEKE 1910, and subsequently observed in all adequately preserved early human embryos in a more or less developed condition. The process attains its maximum development in the embryos Teacher-Bryce II (BRYCE, 1924) and BENEKE, in which it reaches and is attached to the opposite wall of the chorion. The latest stage in which it has been observed in this complete condition is the embryo, G POLITZER, with seven pairs of somites (G. POLITZER, 1930).

The yolk-sac at its postero-dorsal extremity is in the sections seen to be continued into the mesoderm of the connecting stalk in the form of an elongated canal (the allantoic canal). At its caudal end this enlarges to form a distinct thin-walled vesicle, which can be seen. both in the dorsal and lateral views of the embryo (figs. 3 and 4, Plate 29) as a small oval structure on the surface of the connecting stalk close to its attachment to the mesoderm of the chorion, and on the left side of the same 9 (cf. also figs. 28 and 29, Plate 32).

On the surface of the yolk-sac (fig. 3, Plate 29) a number of vessel—primordia can be clearly observed.

In the dorsal view (text-fig. 1 and fig. 4, Plate 29), the shoe—sole shaped form of the embryonal shield is apparent. Along its middle line the head-process and the primitive groove could be made out in the intact specimen. The pointed prolongation of the amniotic cavity along the dorsal surface of the connecting stalk is very distinctly seen in the dorsal view, whilst the amniotic membrane itself, forming the roof of the amnio- embryonal vesicle, is seen in the lateral View (fig. 3, Plate 29).

Embryonal Area

(i) Head-process[1] and Prochordal Plate; - In our preliminary note (HILL-FLORIAN, 1931) we distinguished three portions in the axial preblastoporic structures in our specimen. The most cranial portion we interpreted as a prochordal plate which had already produced a thick mass of prochordal mesoderm. Since the publication of our preliminary note one of us (F.), by the kind permission of Professor HOCHSTETTER, has been able to study the region in question in the embryo Peh.1-Hochstetter (RossEN~BECK, 1923). As this embryo is much better preserved in the region of the prochordal plate than the Dobbin embryo, we have been able to convince ourselves that what we have interpreted as prochordal plate in our preliminary note really represents the cranial end of the head~process. The actual prochordal plate (text-fig. 8) in the Dobbin embryo is very indistinct because of the poor preservation of this region, and it is also possible that the plate varies in difierent embryos. It is represented by an area of thickened endoderm in front of the cranial end of the head—process, presenting no distinct evidence of mesoderm formation. The thickened endoderm of this region can be traced continuously backwards underneath and laterally to the cranial part of the head-process up to section 27. Caudally it becomes indistinct. _ In the head—process itself we can distinguish three parts : (1) A caudal part forming about the caudal half of the entire formation and extending through sections 7 2-46; (2) a shorter part which extends forwards from the caudal segment and gradually widens to pass into continuity with the third part. This second portion we distinguished in our preliminary note as the intermediate region. It is comprised in sections 46 to about 32; (3) a short but broad terminal or cranial part occupying sections 31 to 23.

The caudal part (1) is formed by a typical chorda—canal, its lumen communicating by seven ventral openings with the yolk—sac cavity and by a dorsal opening on the surface of I-IENsEN’s knot with the amnioticcavity. 'I‘he_intermediate part (2) is formed by a segment of the head—process, which here has undergone differentiation into a median chorda~process and lateral mesodermal bands. The most cranial part (3) (which we distinguished as prochordal plate in our preliminary note) is formed by a distinct, dense cellular mass, Olevoid of any differentiation.

The ectoderm of the amnio-embryonal vesicle is first encountered in section 18 and its cavity in section 19 (fig. 9, Plate 30). Between the cranial end of the amnio-embryonal vesicle and the yolk—sac endoderm, there is present a sheet of mesoderm which can be traced cranially up to section 15. . Its cells are fairly compactly arranged and are connected by fine cytodesmata with the ectoderm and endoderm. The median part of this mesodermal sheet passes into continuity behind ‘with the cranial tip of the head—process, which is first visible in section 23. In front of the head-process the prochordal plate appears. The first indication of a thickened endodermal area in the median region seems to occur in section 20, but is only slight. It broadens out in section 21 (fig. 30, Plate 32), and in section 22 becomes more distinct. In section 23 (fig. 31, Plate 32) the prochordal endoderm has almost the same structure as in section 22, the nuclei being in places several deep ; in this section the cranial tip of the head-process appears in the median plane, intercalated into the mesodermal sheet. The head-process rapidly increases in size as it is traced back, and already in section 25 has assumed the appearance seen in section 27 (fig. 10, Plate 30 ; fig. 32, Plate 32). These figures show clearly the mesoderm-like character of this part of the head-process. Laterally on each side, the endoderm is still ‘distinctly thickened, but below the head- process itself it is no longer distinguishable as a separate layer. The cells of the head- process are for the most part compactly arranged and form a mass of considerable thickness, which laterally passes over without definite limit into the adjoining thin sheets of primitive streak mesoderm. The cells contain deeply staining chromatophilic granules which resemble those described by BONNET (1901, p. 287) in the “ Erganzungs- platte ” of the dog, and they are also present in the minute intercellular spaces. In places the cells tend to be arranged round small cavities (one in section 24, two in section 27, fig. 32,Plate 32, and one in section 28, fig. 33, Plate 32), but these vesicles are too indistinct and too irregularly distributed to have any important significance.

The thickened endoderm continued back from the prochordal plate can be traced on either side of the head-process up to section 27 and, as in the preceding sections, is fused with the head-process in the median line. Caudally to this section the thickened endoderm area becomes indistinct, and behind section 34 is certainly no longer present. The possible explanation. of these facts we shall discuss later in connection with the description of the same region in other human embryos (t. p. 482).

In section 30 (fig. 34, Plate 32) the head-process has become reduced, both in thickness and in width, as compared with that of the preceding sections, whilst its cells present a somewhat looser appearance, and the chromatophilic granules have almost completely disappeared. The next section (N o. 31, fig. 35, Plate 32) presents a very similar appearance, except that there are no granules. and the axial mass is more massive.

In both sections no distinct endoderm can be observed below the head-process.

Section 32 (fig. 36, Plate 32) marks the beginning of our intermediate region, since in it we encounter the tip of the chorda-process in the form of a compact circular mass, situated asymmetrically on the left side[2] and composed of cells radially arranged round a potential lumen and possessing peripheral nuclei. On the right of the chorda-process the mesoderm of the head-process forms a dense mass, but on the left it is quite diffuse. There is no clearly defined underlying layer of endoderm.

In the following section (No. 33, fig. 37, Plate 33) the chorda-process is represented by a distinct chorda—canal, with a very obvious lumen and with a mesodermal mass closely adjoining it on each side ; that on the left being more difiuse, that on the right more compact and much less massive than the mass in the corresponding position in the preceding section. Both the masses as well as the chorda-canal closely adjoin the endoderm, which is not distinct as a separate layer. We regard these lateral masses as part of the lateral mesodermal bands, characteristic of the intermediate region.

Similar conditions are observed in section 34 (fig. 38, Plate 33), except that the lumen of the chorda—-canal is much reduced, and in section 35 (fig. 39, Plate 33) only a trace of it remains. The endoderm below the axial region is not distinguishable as a separate layer. This is the case also in section 36 (fig. 40, Plate 33), where the lumen of the chorda—canal again becomes quite distinct.

In section 37 (fig. 41, Plate 33) the chorda-process is solid, and the limits of its ventral wall are difficult to determine. No endoderm is distinguishable below the head—process. The mesodermal bands are distinct, the right being more compact than the left, and are delimited from the chorda—-process dorsally by a groove. Section 38 (fig. 42, Plate 33) is very similar, except that the right mesodermal band is distinctly larger than the left. It is evident that no endoderm is distinct below the head-process. A broad shallow groove (already indicated in the two preceding sections) is now to be found underneath the latter, in contrast to the ventral convexity in the corresponding position in the most cranial region of the head-process. It can be traced back into continuity with the well-marked median groove underlying the caudal half of the head»-process.

In sections 39--43 (figs. 43--46, Plate 33) there is a continuous lumen in the chorda- canal; no definite endoderm is present below the median part of the head-process. The lateral mesodermal bands are symmetrically disposed and quite well marked.

In section 44 (fig. 47, Plate 33) the chorda-canal, broader than in fig. 46, does not show the typical characters seen in the preceding sections. Its lumen appears as a large eccentric cavity of somewhat irregular outline, whilst its cells exhibit an epithelial arrangement only over its dorsal half, those of its ventral wall being irregularly arranged and fused with the endoderm. The lateral mesodermal. bands are now much reduced in size. Essentially the same features are seen in the succeeding section (No. 45, fig. 48, Plate 33). The cells exhibit an epithelial character only on the dorsal side of the solid chorda-process. The right mesodermal band is still distinct, but small, and the left has practically disappeared. In section 46 (fig. 49, Plate 33) the sectional appearance of the axial formation has considerably altered. The chorda-process now appears more massive, owing to the compact groupingof its cells. They show practically no distinct epithelial arrangement, and are fused ventrally with the endoderm, which below the chorda-process is not distinguishable as an independent layer. There may possibly" still be a remnant of the right mesodermal band, but the left has completely disappeared. We may regard this section as marking the junction between our intermediate region and the caudal part of the axial formation, which, as we shall see, consists only‘ of a chorda—canal, devoid of lateral mesodermal bands. In section 47 (fig. 50, Plate 33) the chorda-process is massive and takes the form of a chorda-canal semicircular in outline, the lumen of which appears as a very narrow fissure with a wide opening below into the yolk—sac cavity, and whose wall is composed of cells exhibiting an epithelial arrangement. Marginally, on each side, the wall passes into continuity on the one hand with the lateral (primitive streak) mesoderm, and on the other with the endoderm. We have here in this section the most cranial ventral opening of the chorda-canal, which we shall be able to follow in the succeeding sections.

In section 48 (fig. 51, Plate 33) the chorda-canal is closed; its ventral wall is intercalated in the endoderm, and so forms a part of the dorsal wall of the yolk—sac cavity.

In section 49 a second ventral opening is present in the form of a very narrow slit. The delimitation of the canal from the adjoining mesoderm is not distinct, owing perhaps to the state of. preservation, and this is also the case in the next section (No. 50). In the latter the canal possesses a minute lumen, and this is the case also in sections 51-54 (fig. 12, Plate 30, of section 52).

In sections 55-57 (fig. 13, Plate 30, of section 57) the chorda-canal again opens into the yolk-sac cavity, but in sections 58-59 it is closed, and in section 59 (fig. 14, Plate 30) the endoderm is present below its ventral wall. In section 60 there is a very minute ventral opening, and perhaps also in section 61. In section 62, (fig. 15, Plate 30) the lumen of the canal is distinct, but closed, so also in section 63.

In sections 64 (fig. 16, Plate 31) and 65 the lumen is filled with fine granules, and opens below by a very minute opening. In section 66 the lumen, largely occupied by a nucleus, is closed, and the ventral wall of the canal appears to be fused with the endoderm.

In section 67 the canal, again with a wellemarked lumen, is delimited both from the mesoderm and the underlying endoderm. Its dorsal wall is pointed and projects into the overlying ectoderm. In the following section (No. 68) this dorsal wall is fused with the ectoderm. Its wide lumen, filled with granules of ‘varying size, opens below by a very narrow opening.

In section 69 (fig. 17, Plate 31) the conditions are similar, but the lumen is closed.

In section 70 the conditions are again similar, but the lumen of the canal communicates with the yolk-sac cavity by a definite but very narrow opening, which also is indicated in section 71. We have here in these two sections the ventral opening of a very short neurenteric canal, which runs obliquely upwards and backwards to open into the amniotic cavity by a distinct aperture (blastopore or notopore) in the next section 72 (fig. 18, Plate 31).

Sections 68-7 3 include the region corresponding to HENSEN’s knot, but in this embryo there is no conspicuous thickening marking its position, and only in section 72 is the endoderm closely attached to the ventro-caudal wall of the neurenteric canal. In the other sections in this region the endoderm is separate and clearly distinguishable as an independent layer.

In section 73 (fig. 19, Plate 31) the axial ectoderm exhibits a slight groove prolonged back from the opening of the neurenteric canal, and is continuous below with a compact mass of cells, which represents the caudal wall of that canal. Laterally this mass is continuous with the lateral sheets of primitive streak mesoderm.

That completes the description of the sections through the preblastoporic axial structures in our specimen, and we may now summarise our findings.

The prochordal plate - an area of thickened endoderm - begins close (10 micron.) behind nd behind section 46 (fig. 49, Plate 33), which represents the caudal limit of our intermediate region, are no longer present.

The more caudal msegment of the chorda-canal extends from section 47 back to section 72, and thus has a length of about 208 pi. In this caudal region the chorda- canal is for the most part intercalated in the dorsal wall of the yolk-sac, endoderm being recognisable below it only in a few sections (section 59 and caudally to section 66). The lumen of the canal communicates with the yolk-sac cavity by seven openings, the most cranial of which is situated close behind the caudal limit of the intermediate region, whilst the most caudal forms the ventral opening of a very short neurenteric canal, which passes obliquely upwards and backwards through HENsEN’s knot to open dorsally on section 72, and so connects the yolk-sac cavity with the amniotic cavity.

HENSEN’s knot may be regarded as extending from section 68 - in which the chorda-canal first fuses with the ectoderm - back to section 73. In our embryo it does not form a conspicuous thickening.

The relations of the mesoderm to the chorda-canal may now be reviewed. With the disappearance of the lateral mesodermal bands at the hinder end of the intermediate region, the medial margins of the lateral sheets of primitive streak mesoderm, which are not specially thickened, come into direct relationship with the lateral margins of the chorda-canal, and in many of the sections that relationship is so intimate as to suggest actual continuity between the two. Indeed in some sections (fig. 15, Plate 30, and fig. 51, Plate 33) continuity appears to be mediated by actual extensions of the wall of the chorda-canal, so that a direct (if small) contribution from the head-process of this region to the lateral mesoderm cannot be entirely excluded. That possibility raises the question whether the “ lateral mesodermal bands ” of the intermediate region may not also be derivatives of the head-process of that region. Apart from the obvious close topographical relationship between these bands and the median chorda-process, intensive study of the sections failed to yield any positive evidence of actual genetic relationship between them, and we were consequently much exercised as to the inter- pretation we should put upon these bands.

We had examined sections of the corresponding region in other mammalian embryos without gaining much enlightenment, when, fortunately, the sections of a very well- preserved presomite embryo of Loris (Loris 49 of the collection made by Professor A. SUBBA RAU, the embryonal area of which measures 1-92 >< 1-25 mm.) became available for study, and proved invaluable for comparison with those of the Dobbin embryo. Loris 49 is in much the same developmental stage as the latter, so far as concerns the head~process, but is a little more advanced, inasmuch as the caudal portion of the chorda-canal has already become transformed into the definitive chorda-plate. Cranially, however, the chorda-canal is still intact, and a region corresponding to the intermediate region of the Dobbin embryo is clearly recognisable, whilst a well-marked prochordal plate region is also present. A detailed description of this very interesting embryo will begiven elsewhere. Here we reproduce, as figs. 55-59, Plate 34, five photomicrographs through the most important regions of the preblastoporic axial formation, for comparison with our figures of the Dobbin embryo. Fig. 59, Plate 34;, shows the chorda—canal (about 0 - 19 mm.) in front of the cranial extremity of the chorda-plate It shows the typical characters. The lumen is transversely wide, and the dorsal wall is distinctly thicker and more definitely epithelial in nature than the ventral, which is closely connected with the underlying endoderm. On either side of the chorda—canal and fairly definitively delimited from it, is a thick sheet of primitive streak mesoderm.

The section here figured may be compared with the sections through the caudal region. of the chorda—canal in the Dobbin embryo, in which no ventral openings are present (figs. 12, 15, Plate 30, and fig. 16, Plate 31).

Fig. 58, Plate 34, is 0-176 mm. (22 sections) in front of fig. 59, and represents a section through the caudal portion of the intermediate region. Here the axial formation is clearly distinguishable into three parts - a median chorda—canal, and two smaller luminated masses situated symmetrically one on either side of the canal and in continuity with its lateral wall. These masses, as the serial sections show, are not isolated structures, but, as in the Dobbin embryo, are simply sections of two continuous cords or bands of mesoderm which accompany the chorda—canal throughout the extent of the inter- mediate region. We regard them as the homologues of the “lateral mesodermal bands" of that embryo.

The lateral limits of the chorda—canal are somewhat indefinite, owing to its continuity with the lateral masses, but it appears to be of much the same width as in fig. 59, though its lumen is smaller. Its ventral wall also lacks a definite ventral boundary, since it is connected with the underlying endoderm 3 by somewhat loosely arranged mesenchymatous cells. The lateral masses in this section are very striking structures. Each

possesses a very small central lumen, round which the enclosing cells are arranged. in a radiating fashion, after the manner of an epithelium, thus recalling the mesodermal somites of an early embryo. It is deserving of emphasis that they are in direct continuity with the chorda—canal, whilst they are very definitely delimited from the lateral sheets of primitive streak mesoderm, which are here thinner than in fig. 59. Comparison of fig. 58 with fig. 46, Plate. 33, through precisely the same region -of the Dobbin embryo, reveals at once the striking agreement between the two, the only difference being that, in the Dobbin embryo, the lateral bands are solid and less highly differentiated than in Loris 49, and are in apparent continuity with the lateral sheets of primitive streak mesoderm.

Fig. 57, Plate 34, is thirteen sections in front of fig. .58. We can still recognise the median chorda—canal with a relatively small lumen and the two lateral bands, also, with slit-like lumina. Although in this section the lumen of the chorda—canal is small, in other sections it may be duplicated and may even be prolonged towards the lumina in the lateral bands. In this region the axial structures (chorda—canal and lateral mesodermal bands) are underlain by a thick zone of ‘cells, loosely arranged immediately underneath the chorda—canal, but becoming more compact below. The lowermost stratum of cells of this more compact zone directly bounds the yolk-sac cavity, so that the endoderm is not distinguishable as a separate and independent layer. Essentially similar conditions are found also in the Dobbin embryo (cf. p. 453), but in the latter the lowermost stratum of cells is not so distinctly thickened. The appearances suggest that the axial endoderm has here undergone active proliferation, and accordingly it might be regarded as a part of the prochordal plate here overlapped by the head- process. On the other hand, it may be that the cellular zone in question is to be regarded simply as the ventral part of the original head-process here indistinguishably fused with the endoderm. This latter view seems to us the more probable (of. p. 482).

Our next figure through this Loris embryo (fig. 56, Plate 34) shows a section through the cranial extremity of the head-process, where it is quite undifferentiated and much narrower than in fig. 57. The cell-layer underlying the head-process is not so thick as in the latter figure, and shows a greater tendency to an epithelial arrangement, though it still presents the appearance of proliferation. In this figure it can be seen that the marginal portion of the embryonal endoderm situated on either side of the axial region has the form of a well-defined thick layer composed of columnar cells. Traced medially this thick endoderm passes into a more or less definitely localised cell—mass, which projects upwards from the endoderm, and on the left side appears like a fold of the latter, since there is present in it an indistinct cleft—like lumen. We have to do here with two paraxial thickenings in which mesoderm is being proliferated from endoderm. These thickenings are also visible in fig. 57 and on the left side in fig. 58, and they can be traced forwards from fig. 56 into continuity with the prochordal ‘plate, so that in reality they have the form of paraxial bands, connected cranially by the pro- chordal plate.

Our last figure through this embryo (fig. 55, Plate 34) passes through the middle of the prochordal plate, which is readily distinguishable from the columnar embryonal endoderm adjoining its lateral margins by its more irregular character, and by the fact that it consists of a thinner axial portion uniting two distinct lateral thickenings. These latter are similar in character to the paraxial bands noted in fig. 56. They can be traced caudally in the series into direct continuity with them, whilst cranially the prochordal plate loses its bilateral character and appears as a single thickening.

The relations of the lateral mesodermal bands to the chorda—canal in this Loris embryo, as briefly set forth above, lead us to the conclusion that the cranial segment of the head-process in the Loris embryo, as in the human, undergoes differentiation in the caudo-cranial direction into a median chorda—canal and two lateral mesodermal bands. On the assumption that the head-process is archenteric (metenteric) in significance, these bands may be regarded as representing cephalic mesoderm of direct archenteric (metenteric) origin (so-called “ gastra ” mesoderm). It is perhaps not without significance that the Lemur embryo in this matter of the lateral mesodermal bands should provide such an exact parallel to the condition prevailing in the early human embryo.

Whilst in the Loris and Dobbin embryos, the lateral bands of “ gastral ” mesoderm are confined to the cranial region of the head-process, it is of considerable interest to recall that WILSON and HILL (1907) have shown that in early embryos of Ornitho- rhynchus, the caudal portion of the archenteric plate (head-process) extending forwards from the primitive (HENsEN’s) knot, is distinguishable into a median portion destined to form the chorda—plate and thickened lateral portions which lie in series with the lateral masses (“ protosomites ”) of the primitive knot and which are destined to become converted into “ gastral ” mesoderm. As may be seen from WILSON and HILL’s figures (of. especially figs. 63-66, Plate 12), these lateral portions of the Ornithorhynchus plate are strikingly similar to our lateral mesodermal bands in their relations. The facts suggest that the head-process was originally capable of producing “ gastral ” mesoderm along its entire length.

(ii) Mesoderm, Primitive Streak and Cloacasl M embmiie.-Laterally the most cranial part of the head-process passes on each side into continuity with a mesodermal sheet situated between the shield-ectoderm and the yolk-sac endoderm and attached marginally to the mesothelial investment of the amnio-embryonal and yolk-sac vesicles. The limits between the head-process and the said mesodermal sheets are not sharp enough to permit of the precise delimitation of the head-process, but the latter probably does not extend laterally beyond the limiting lines indicated in text-figs. 1 and 8.

The existence of the “ primary ” or extra—embryonal mesoderm in the human embryo makes the analysis of the mesoderm situated laterally to the axial structures some- what difiicult. As the cells of the “ primary ” mesoderm differ in no important respect from those of the later formed primitive streak mesoderm, the main criterion we can use as a guide in distinguishing between the two is the relative density of the layers. T

We have mentioned already (p. 449) that underneath the cranial margin of the shield- ectoderm there is a mesoderm plate of such density that it can hardly be regarded as composed only of “primary ” mesoderm ; it is more probable that the primitive streak mesoderm has already reached the cranial border of the embryonal shield, and par- ticipates in the formation of the plate in question. If we follow this plate caudally we notice a curious asymmetry in the density of the plate on the two sides. On the left side (right in the embryo) the mesoderm plate is much less compact than on the right. This difference in thickness reaches approximately to section 50 in the caudal direction, i.c., a little more caudally than the caudal limit of the intermediate region (section 46). We have no other explanation of this asymmetry to offer than to suggest that it is simply an individual variation. We tried at first to use some of the breaks in the mesodermal sheets for the determination of the probable limit reached by the primitive streak mesoderm, but finally came to the conclusion that the primitive streak mesoderm on both sides extends forwards in front of the prochordal plate.

Caudally to the region of HENsEN’s knot the artificial dorsal concavity of the embryonal shield gradually diminishes, and behind section 111 (fig. 23, Plate 31) the embryonal shield assumes its normal dorsally convex form. The primitive streak is long (420 p. inlength, measured in theimedian section along the curvature) and is well developed. It extends from section 741 back‘ to the cloacal membrane, and from section 90 (over its posterior «two-thirds) a distinct primitive groove is distinguishable along it (figs. 20——23, "Plate 31). The lateral sheets of mesoderm. arising from it are thick and compact, though in some sections the cells tend to be arranged in two layers. The sheets extend beyond the margins of the embryonal shield on both sides to become continuous with the mesothelial covering of theramnio-—em.bryonal and yolk-sac. vesicles. In certain sections (figs. 20 and 21, Plate 31) a small cavity is present in the sheet at its point of attachment to the mesothelium, the significance of which is doubtful (perhaps ccelomic or vascular). The endoderm is clearly distinguishable below the whole extent of the primitive streak.

At the caudal end of the primitive streak its ectoderm passes over into that of the cloacal membrane. The transition is very gradual, so that the determination of the cranial limit of the cloacal membrane is somewhat diflicult. We may observe that the cell-proliferation from the bottom * of the primitive groove - which is very clearly marked (figs. 22 and 23, Plate 31) - gradually disappears, whilst the ectoderm thickens and grows down towards the endoderm, and finally comes into contact with the latter (section 117, fig. 52, Plate 34). It is difficult to determine whether in section 117 we have an actual cloacal membrane or not. The ectoderm is certainly in connection with the endoderm, whilst no connection with the mesoderm seems to exist, except latero-dorsally on the apparent right side, but the fusion of the ectoderm and endoderm is not so complete as it ought to be in a fully developed cloacal membrane. The conditions seen here can be explained if we assume that the cloacal membrane extends in the cranial direction partly at the cost of the primitive streak. In the following sections (118--120) the connection between the two embryonal layers is less complete than in section 117, but there are present in these sections chromatophilic granules, which BONNET (1901) first recorded as of common occurrence in this region as well as in the prochordal plate, and which have been found in human embryos of later stages by STERNBERG (1927) and FLORIAN (1930, cc). These two authors regarded these granules as products’ of cell~dissolution, but, as indicated on p. 453, they have possibly quite a different significance.

In section 121 (fig. 24, Plate 32) the mesoderm on both sides is clearly separated from the ectoderm in the median plane, and a typical cloacal membrane is certainly present ; but the fusion between the ectoderm and endoderm is not yet so complete as in section 122 (fig. 53, Plate 34). In both sections the above-mentioned granules are present in large numbers in the cloacal membrane and in the following sections (123—-125, fig. 54, Plate 34), also in the mesoderm quite close to. the cloacal membrane. In these latter sections the fusion of the ectoderm and the endoderm becomes less marked ; whether there are still traces of fusion in sections 126-127 (fig. 25, Plate 32) is difficult to determine. s In section 128 (fig. 26, Plate 32) ecto- and endoderm are widely separated.

It is worthy of note that a groove in continuity with the primitive groove is present along the dorsal surface of the cloacal membrane in our specimen. FLORIAN (1930, a) has also observed a corresponding groove in embryo Bi II.

  1. We employ the old term “ head—process ” for the axial formation which extends forwards from I-IENsEN’s knot, the homologue of the dorsal lip of the blastopore (notopore). This formation comprises not only the primordium of a part of the mesoderm (which does not seem to be very extensive), but also that of the chorda, and therefore such terms as “ Mesodermsackchen ” (HERTWIG, 0., 1906) or “ kranialer Mesoblastfortsatz ” (WALDEYER, 1929) are not suitable.
  2. Right and left in the description of the figures correspond to the reader’s right and left, but‘ actually to the left and right sides of the embryo respectively, 6.6., the apparent right side of the figure is the actual left in the embryo and vice versa. 3


We have already described (p. 448) the form of the yolk-sac, and here we need only give a very brief account of the structure of the yolk-sac wall. We have dealt with the endoderm. in the median part of the dorsal wall in our description of the axial structures of the embryonal area. In the lateral parts of the dorsal wall the endoderm is composed of a single layer of flattened cells, which in this region is for the most part artificially detached from the mesoderm. In places where this detachment has not taken place, it is connected with the mesoderm by means of cytodesmata. In the dorsal part of the lateral wall the endoderm is also thin, but in the more ventral region it is composed of a well-marked layer of cubical or low columnar cells, the limits of which are distinct (figs. 5, 7 and 8, Plate. 29).

In the lateral and ventral portions of the wall the formation of blood-vessels is in active progress. The vessel-primordia range from solid blood-islands to vessels, each with an endothelial wall enclosing a lumen more or less completely filled by primitive blood-cells. Vessel development is more advanced in the ventral than in the more dorsal I parts of the wall, the endothelial vessels in that region often possessing large lumina, only partly filled by blood-cells. p The vessel-lumina do not as yet form a continuous network.

The preservation of our specimen is not sufficiently perfect to permit us to discuss the origin of the vascular elements.

Allantoic Canal

As is shown in our reconstruction (text-figs. 1 and 2) the yolk-sac at its caudo-dorsal extremity gives origin to a long tubular diverticulum which extends into the mesenchyme of the connecting stalk. This is the primordium of the allantoic canal. It is situated caudally to the cloacal membrane, but, as in other described embryos of about this stage, it arises not from the hind-gut as in lower Amniotes, but from that part of the yolk-sac which is destined in later stages to form the hind-gut. FLORIAN (1930, ct) has proposed to term the diverticulum of the yolk—sac~—-——which is destined to furnish not only the allantoic canal, but also part of the hind-g‘ut———-the “ allanto—enteric diverticulum ” (cf. FLonIAN’s text—figs. 6 and 7).

In our specimen the di verticulum referred to above represents the primordium of the allantoic canal alone ; it is situated behind the cloacal membrane, which is not included in it. It is distinguishable into two parts, viz., an elongated tubular cranial part and a swollen vesicular terminal part, already noted in our description of the

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Cite this page: Hill, M.A. (2021, June 21) Embryology Paper - A Young Human Embryo (Embryo Dobbin) with Head-Process and Prochordal Plate. Retrieved from

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