Illustration: Figure 96Fig. 96. Surface view of the area pellucida of a chick’s blastoderm shortly after the formation of the primitive groove.pr.primitive streak with primitive groove;af.amniotic fold.The darker shading round the primitive streak shews the extension of the mesoblast.
Fig. 96. Surface view of the area pellucida of a chick’s blastoderm shortly after the formation of the primitive groove.pr.primitive streak with primitive groove;af.amniotic fold.The darker shading round the primitive streak shews the extension of the mesoblast.
In the course of further growth the area pellucida soon becomes pyriform, the narrower extremity being the posterior. The primitive streak (fig. 96) elongates considerably, so as to occupy about two-thirds of the length of the area pellucida; but its hinder end in many instances does not extend to the posterior border of the area pellucida. The median line of the primitive streak becomes marked by a shallow groove, known as theprimitive groove.
Illustration: Figure 97Fig. 97. Transverse section through the front end of the primitive streak of a blastoderm of the same age as fig. 96.pv.primitive groove;m.mesoblast;ep.epiblast;hy.hypoblast;yh.yolk of germinal wall.
Fig. 97. Transverse section through the front end of the primitive streak of a blastoderm of the same age as fig. 96.pv.primitive groove;m.mesoblast;ep.epiblast;hy.hypoblast;yh.yolk of germinal wall.
During these changes in external appearance there grow from the sides of the primitive streak two lateral wings of mesoblast cells, which gradually extend till they reach the sides of the area pellucida (fig. 97). The mesoblast still remains attached to the epiblast along the line of the primitive streak. During this extension many sections through the primitive streak give an impression of the mesoblast being involuted at the lips of a fold, and so support the view above propounded, that the primitive streak is the rudiment of the coalesced lips of the blastopore. The hypoblast below the primitive streak is always quite independent of the mesoblast above, though much more closely attached to it in the median line than at the sides. The part of the mesoblast, which I believe to be derived from the primitive hypoblast, can generally be distinctly traced. In many cases, especially at the front end of the primitive streak, it forms, as infig. 97, a distinct layer of stellate cells, quite unlike therounded cells of the mesoblastic involution of the primitive streak.
Illustration: Figure 98Fig. 98. Longitudinal section through the axial line of the primitive streak, and the part of the blastoderm in front of it, of an embryo chick somewhat younger than fig. 99.pr.s.primitive streak;ep.epiblast;hy.hypoblast of region in front of primitive streak;n.nuclei;yk.yolk of germinal wall.
Fig. 98. Longitudinal section through the axial line of the primitive streak, and the part of the blastoderm in front of it, of an embryo chick somewhat younger than fig. 99.pr.s.primitive streak;ep.epiblast;hy.hypoblast of region in front of primitive streak;n.nuclei;yk.yolk of germinal wall.
In the region in front of the primitive streak, where the first trace of the embryo will shortly appear, the layers at first undergo no important changes, except that the hypoblast becomes somewhat thicker. Soon, however, as shewn in longitudinal section infig. 98, the hypoblast along the axial line becomes continuous behind with the front end of the primitive streak. Thus at this point, which is the future hind end of the embryo, the mesoblast, the epiblast, and the hypoblast all unite together; just as they do in all the types of Ichthyopsida.
Shortly afterwards, at a slightly later stage than that represented infig. 96, an important change takes place in the constitution of the hypoblast in front of the primitive streak. The rounded cells, of which it is at first composed (fig. 98), break up into (1) a layer formed of a single row of more or less flattened elements below—the hypoblast—and (2) into a layer formed of several rows of stellate elements, between the hypoblast and the epiblast—the mesoblast (fig. 99). A separation between these two layers is at first hardly apparent, and before it has become at all well marked, especially in the median line, an axial opaque line makes its appearance in surface views, continued forwardsfrom the front end of the primitive streak, but stopping short at a semicircular fold—the future head-fold—near the front end of the area pellucida. In section (fig. 100) this opaque line is seen to be due to a special concentration of cells in the form of a cord. This cord is the commencement of the notochord (ch). In some instances the commencing notochord remains attached to the hypoblast, while the mesoblast is laterally quite distinct (videfig. 100), and is therefore formed in the same manner as in most Ichthyopsida; while in other instances, and always apparently in the Goose (Gasser,No.127), the notochord appears to become differentiated in the already separated layer of mesoblast. In all casesthe notochord and the hypoblast below it unite with the front end of the primitive streak; with which also the two lateral plates of mesoblast become continuous.
Illustration: Figure 99Fig. 99. Transverse section through the embryonic region of the blastoderm of a Chick shortly prior to the formation of the medullary groove and notochord.m.median line of the section;ep.epiblast;ll.lower layer cells (primitive hypoblast) not yet completely differentiated into mesoblast and hypoblast;n.nuclei of germinal wall.
Fig. 99. Transverse section through the embryonic region of the blastoderm of a Chick shortly prior to the formation of the medullary groove and notochord.m.median line of the section;ep.epiblast;ll.lower layer cells (primitive hypoblast) not yet completely differentiated into mesoblast and hypoblast;n.nuclei of germinal wall.
From what has just been said it is clear that in the region of the embryo the mesoblast originates as two lateral plates split off from the hypoblast, and that the notochord originates as a median plate, simultaneously with the mesoblast, with which it may sometimes be at first continuous.
Kölliker holds that the mesoblast of the region of the embryo is derived from a forward growth from the primitive streak. There is no theoretical objection to this view, and I think it would be impossible to shew for certain by sections whether or not there is a growth such as he describes; but such sections as that represented infig. 99(and I have series of similar sections from several embryos) appear to me to be conclusive in favour of the view that the mesoblast of the region of the embryo is to a large extent derivedfrom a differentiation of the primitive hypoblast. I am however inclined to believe that some of the mesoblast cells of the embryonic region have the derivation which Kölliker ascribes to all of them.
Illustration: Figure 100Fig. 100. Transverse section through the embryonic region of the blastoderm of a Chick at the time of the formation of the notochord, but before the appearance of the medullary groove.ep.epiblast;hy.hypoblast;ch.notochord;me.mesoblast;n.nuclei of the germinal wallyk.yolk.
Fig. 100. Transverse section through the embryonic region of the blastoderm of a Chick at the time of the formation of the notochord, but before the appearance of the medullary groove.ep.epiblast;hy.hypoblast;ch.notochord;me.mesoblast;n.nuclei of the germinal wallyk.yolk.
As regards the mesoblast of the primitive streak, in a purely objective description like that given above, the greater part of it may fairly be described as being derived from the epiblast. But if it is granted that the primitive streak corresponds with the blastopore, it is obvious to the comparative embryologist that the mesoblast derived from it really originates from the lips of the blastopore, as in so many other cases; and that to describe it, without explanation, as arising from the epiblast, would give an erroneous impression of the real nature of the process.
Illustration: Figure 101Fig. 101. Transverse section of a blastoderm incubated for 18 hours.The section passes through the medullary groovemc., at some distance behind its front end.A. epiblast. B. mesoblast. C. hypoblast.m.c.medullary groove;m.f.medullary fold;ch.notochord.
Fig. 101. Transverse section of a blastoderm incubated for 18 hours.The section passes through the medullary groovemc., at some distance behind its front end.A. epiblast. B. mesoblast. C. hypoblast.m.c.medullary groove;m.f.medullary fold;ch.notochord.
The differentiation of the embryo may be said to commence with the formation of the notochord and the lateral plates of mesoblast. Very shortly after the formation of these structures the axial part of the epiblast, above the notochord and in front of the primitive streak, which is somewhat thicker thanthe lateral parts, becomes differentiated into a distinct medullary plate, the sides of which form two folds—the medullary folds—enclosing between them a medullary groove (fig. 101).
In front the two medullary folds meet, while posteriorly they thin out and envelop between them the front end of the primitive streak. On the formation of the medullary folds the embryo assumes a form not unlike that of the embryos of many Ichthyopsida at a corresponding stage. The appearance of the embryo, and its relation to the surrounding parts is somewhat diagrammatically represented infig. 102. The primitive streak now ends with an anterior swelling (not represented in the figure), and is usually somewhat unsymmetrical. In most cases its axis is more nearly continuous with the left, or sometimes the right, medullary fold than with the medullary groove. In sections its front end appears as a ridge on one side or on the middle of the floor of the widened end of the medullary groove.
Illustration: Figure 102Fig. 102. Surface view of the pellucid area of a blastoderm of 18 hours.None of the opaque area is shewn, the pear-shaped outline indicating the limits of the pellucid area.At the hinder part of the area is seen the primitive groovepr., with its nearly parallel walls, fading away behind, but curving round and meeting in front so as to form a distinct anterior termination to the groove, about halfway up the pellucid area.Above the primitive groove is seen the medullary groovem.c., with the medullary foldsA.These, diverging behind, slope away on either side of the primitive groove, while in front they curve round and meet each other close upon a curved line which represents the head-fold.The second curved line in front of and concentric with the first is the commencing fold of the amnion.
Fig. 102. Surface view of the pellucid area of a blastoderm of 18 hours.None of the opaque area is shewn, the pear-shaped outline indicating the limits of the pellucid area.At the hinder part of the area is seen the primitive groovepr., with its nearly parallel walls, fading away behind, but curving round and meeting in front so as to form a distinct anterior termination to the groove, about halfway up the pellucid area.Above the primitive groove is seen the medullary groovem.c., with the medullary foldsA.These, diverging behind, slope away on either side of the primitive groove, while in front they curve round and meet each other close upon a curved line which represents the head-fold.The second curved line in front of and concentric with the first is the commencing fold of the amnion.
The mesoblast and hypoblast, within the area pellucida, do not give rise to the whole of these two layers in the surrounding area opaca; but the whole of the hypoblast of the area opaca, and a large portion of the mesoblast, and possibly even some of the epiblast, take their origin from the peculiar material already spoken of, which forms the germinal wall, and is continuous withthe hypoblast at the edge of the area opaca (videfigs.91,94,97,98,99,100).
The exact nature of this material has been the subject of many controversies. Into these controversies it is not my purpose to enter, but subjoined are the results of my own examination. The germinal wall first consists, as already mentioned, of the lower cells of the thickened edge of the blastoderm, and of the subjacent yolk material with nuclei. During the period before the formation of the primitive streak the epiblast extends itself over the yolk, partly, it appears, at the expense of the cells of the germinal wall, and possibly even of cells formed around the nuclei in this part. This mode of growth of the epiblast is very similar to that in the epibolic gastrulas of many Invertebrata, of the Lamprey, etc.; but how far this process is continued in the subsequent extension of the epiblast I am unable to say. The cells of the germinal wall, which are at first well separated from the yolk below, become gradually absorbed in the growth of the hypoblast, and the remaining cells and yolk then become mingled together, and constitute a compound structure, continuous at its inner border with the hypoblast. This structure is the germinal wall usually so described. It is mainly formed of yolk granules with numerous nuclei, and a somewhat variable number of largish cells imbedded amongst them. The nuclei typically form a special layer immediately below the epiblast, some of which are probably enclosed by a definite cell-body. A special mass of nuclei (videfigs.98and100,n) is usually present at the junction of the hypoblast with the germinal wall.
The germinal wall at this stage corresponds in many respects with the granular material, forming a ring below the edge of the blastoderm in Teleostei.
It retains the characters above enumerated till near the close of the first day of incubation,i.e.till several mesoblastic somites have become established. It then becomes more distinctly separated from the subjacent yolk, and its component parts change very considerably in character. The whole wall becomes much less granular. It is then mainly formed of large vesicles, which often assume a palisade-like arrangement, and contain granular balls, spherules of white yolk, and in an early stage a good deal of granular matter (videfig. 115). These bodies have some resemblance to cells, and have been regarded as such by Kölliker (No.135) and Virchow (No.150): they contain however nothing which can be considered as a nucleus. Between them however nuclei[62]may easily be seen in specimens hardened in picric acid, and stained with hæmatoxylin (these nuclei are not shewn infig. 115). These nuclei are about the same size as those of the hypoblast cells, and are surrounded by a thin layer of granular protoplasm,which is continuous with a mesh-work of granular protoplasm enveloping the above described vesicles. The germinal wall is still continuous with the hypoblast at its edge; and close to the junction of the two the hypoblast at first forms a layer of moderately columnar cells, one or two deep and directly continuous with the germinal wall, and at a later period usually consists of a mass of rounder cells lying above the somewhat abrupt inner edge of the germinal wall.
The germinal wall certainly gives rise to the hypoblast cells, which mainly grow at its expense. They arise at the edge of the area pellucida, and when first formed are markedly columnar, and enclose in their protoplasm one of the smaller vesicles of the germinal wall.
In the later stages (fourth day and onwards) the whole germinal wall is stated to break up into columnar hypoblast cells, each of them mainly formed of one of the vesicles just spoken of. After the commencing formation of the embryo the mesoblast becomes established at the inner edge of the area opaca, between the germinal wall and the epiblast; and gives rise to the tissue which eventually forms the area vasculosa. It seems probable that the mesoblast in this situation is mainly derived from cells formed around the nuclei of the germinal wall, which are usually specially aggregated close below the epiblast. Disse (No.122) has especially brought evidence in favour of this view, and my own observations also support it.
The mesoblastic somites begin to be formed in the lateral plates of the mesoblast before the closure of the medullary folds. The first somite arises close to the foremost extremity of the primitive streak, but the next is stated to arise in front of this, so that the first formed somite corresponds to the second permanent vertebra[63]. The region of the embryo in front of the second formed somite—at first the largest part of the embryo—is the cephalic region. The somites following the second are formed in the regular manner, from before backwards, out of the unsegmented posterior part of the embryo, which rapidly grows in length to supply the necessary material (fig. 103). As the somites retain during the early stages of development an approximately constant breadth, their number is a fair test of the length of the trunk. With the growth of the embryo the primitive streak is continually carried back, the lengthening of the embryo always taking place between the front end of the primitive streak and the last somite; and during thisprocess the primitive streak undergoes important changes both in itself and in its relation to the embryo. Its anterior thicker part, which is enveloped in the diverging medullary folds, soon becomes distinguished in structure from the part behind this, and placed symmetrically in relation to the axis of the embryo (fig. 103,a.pr), and at the same time the medullary folds, which at first simply diverge on each side of the primitive streak, bend in again and meet behind so as completely to enclose the front part of the primitive streak. The region of the embryo bird, where the medullary folds diverge, is known as the sinus rhomboidalis, though it has no connection with the similarly named structure in the adult. By the time that ten somites are formed the sinus rhomboidalis is completely established, and the medullary groove has become converted into a tube till close up to the front end of the sinus. In the following stages the closure of the medullary canal extends to the sinus rhomboidalis, and the folding off of the hind end of the embryo from the yolk commences. Coincidently with the last-named changes the sides of the front part of the primitive streak become thickened, and give rise to conspicuous caudal swellings; in which the layers of the embryo are indistinguishably fused. The apparently hinder part of the primitive streak becomes, as more particularly explained in the sequel, folded downwards and forwards on the ventral side.
Illustration: Figure 103Fig. 103. Dorsal view of the hardened blastoderm of a Chick with five mesoblastic somites. The medullary folds have met for part of their extent, but have not united.a.pr.anterior part of the primitive streak;p.pr.posterior part of the primitive streak.
Fig. 103. Dorsal view of the hardened blastoderm of a Chick with five mesoblastic somites. The medullary folds have met for part of their extent, but have not united.a.pr.anterior part of the primitive streak;p.pr.posterior part of the primitive streak.
This is a convenient place to notice remarkable appearances which present themselves close to the junction of the neural plate and the primitive streak. These are temporary passages leading from the hinder end of the neural tube into the alimentary canal. They vary somewhat in different species of birds, and it appears that in the same species there may be several openings of the kind, which appear one after the other and thenclose again. They were first discovered by Gasser (No.127). In all cases[64]they lead round the posterior end of the notochord, or through the point where the notochord falls into the primitive streak.
If the primitive streak is, as I believe, formed of the lips of the blastopore, there can be but little doubt that these structures are disappearing, and functionless rudiments of the opening of the blastopore, and they thus lend support to my view as to the nature of the primitive streak. That, in part, they correspond with the neurenteric canal of the Ichthyopsida is clear from the detailed statements below. Till their relations have been more fully worked out it is not possible to give a more definite explanation of them.
According to Braun (No.120) three independent communications are to be distinguished in Birds. These are best developed in the Duck. The first of these is a small funnel-shaped diverticulum leading from the neural groove through the hypoblast. It is visible when eight mesoblastic somites are present, and soon disappears. The second, which is the only one I have myself investigated, is present in the embryo duck with twenty-six mesoblastic somites, and is represented in the series of sections (fig. 104). The passage leads obliquely backwards and ventralwards from the hind end of the neural tube into the notochord, where the latter joins the primitive streak (B). A narrow diverticulum from this passage is continued forwards for a short distance along the axis of the notochord (A,ch). After traversing the notochord, the passage is continued into a hypoblastic diverticulum, which opens ventrally into the future lumen of the alimentary tract (C). Shortly behind the point where the neurenteric passage communicates with the neural tube the latter structure opens dorsally, and a groove on the surface of the primitive streak is continued backwards from it for a short distance (C). The first part of this passage to appear is the hypoblastic diverticulum above mentioned.
This passage does not long remain open, but after its closure, when the tail-end of the embryo has become folded off from the yolk, a third passage is established, and leads round the end of the notochord from the closed medullary canal into the postanal gut. It is shewn diagrammatically infig. 106,ne, and, as may be gathered from that figure, has the same relations as the neurenteric canal of the Ichthyopsida.
In the goose a passage has been described by Gasser, which appears when about fourteen or fifteen somites are present, and lasts till twenty-three are formed. Behind its opening the medullary canal is continued back as a small diverticulum, which follows the course of the primitive groove and is apparently formed by the conversion of this groove into a canal. It is at first open to the exterior, but soon becomes closed, and then atrophies.
Illustration: Figure 104Fig. 104. Four transverse sections through the neurenteric passage and adjoining parts in a Duck embryo with twenty-six mesoblastic somites.A. Section in front of the neurenteric canal shewing a lumen in the notochord.B. Section through the passage from the medullary canal into the notochord.C. Section shewing the hypoblastic opening of the neurenteric canal, and the groove on the surface of the primitive streak, which opens in front into the medullary canal.D. Primitive streak immediately behind the opening of the neurenteric passage.mc.medullary canal;ep.epiblast;hy.hypoblast;ch.notochord;pr.primitive streak.
Fig. 104. Four transverse sections through the neurenteric passage and adjoining parts in a Duck embryo with twenty-six mesoblastic somites.A. Section in front of the neurenteric canal shewing a lumen in the notochord.B. Section through the passage from the medullary canal into the notochord.C. Section shewing the hypoblastic opening of the neurenteric canal, and the groove on the surface of the primitive streak, which opens in front into the medullary canal.D. Primitive streak immediately behind the opening of the neurenteric passage.mc.medullary canal;ep.epiblast;hy.hypoblast;ch.notochord;pr.primitive streak.
In the chick there is a perforation on the floor of the neural canal,which is not so marked as those in the goose or duck, and never results in a complete contin164uity between the neural and alimentary tracts; but simply leads from the floor of the neural canal into the tissues of the tail swelling, and thence into a cavity in the posterior part of the notochord. The hinder diverticulum of the neural canal along the line of the primitive groove is, moreover, very considerable in the chick, and is not so soon obliterated as in the goose. The incomplete passage in the chick arises when about twelve somites are present. It is regarded by Braun as equivalent to the first formed passage in the duck, but I very much doubt whether there is a very exact equivalence between the openings in different types, and think it more probable that they are variable remnants of a primitive neurenteric canal, which in the ancestors of those forms persisted through the whole period of the early development. The third passage is formed in the chick (Kupffer) during the third day of incubation. InMelopsittacus undulatus the two first communications are stated by Braun (No.120) to be present at the same time, the one in front of the other.
It is probable, from the above description, that the front portion of the primitive streak in the bird corresponds with that part of the lips of the blastopore in Elasmobranchii which becomes converted into the tail swelling and the lining of the neurentic canal; while the original groove of the front part of the primitive streak appears to be converted into the posterior diverticulum of the neural canal. The hinder part of the primitive streak of the bird corresponds, in a very general way, with the part of the blastopore in Elasmobranchii, which shuts off the embryo from the edge of the blastoderm (videp.64), though there is of course no genetic relation between the two structures. When the anterior part of the streak is becoming converted into the tail swelling, the groove of the posterior part gradually shallows and finally disappears. The hinder part itself atrophies from behind forwards, and in the course of the folding off of the embryo from the yolk the part of the blastoderm where it was placed becomes folded in, so as to form part of the ventral wall of the embryo. The apparent hinder part of the primitive streak is therefore in reality the ventral and anterior part[65].
It has generally been maintained that the primitive streak and groove become wholly converted into the dorsal portion of the trunk of the embryo,i.e.into the posterior part of the medullary plate and subjacent structures. This view appears to me untenable in itself, and quite incompatible with the interpretation of the primitive streak given above. To shew how improbable it is, apart from any theoretical considerations, I have compiled two tables of the relative lengths of the primitive streak and the body of the embryo, measured by the number of sections made through them, in a series of examples from the data in Gasser’s important memoir (No.127). In these tables each horizontal line relates to a single embryo. The first column shews the number of somites, and the second the number of sectionsthrough the primitive streak. Where the primitive streak becomes divided into two parts the sections through the two parts are given separately: the left column (A) referring to the anterior part of the streak; the right column (P) to the posterior part. The third column gives the number of sections through the embryo. The first table is for fowl embryos, the second for goose embryos.
An inspection of these two tables shews that an actual diminution in the length of the primitive streak takes place just about the time when the first somites are being formed, but there is no ground for thinking that the primitive streak becomes then converted into the medullary plate. Subsequently the primitive streak does not for a considerable time become markedly shorter, and certainly its curtailment is not really sufficient to account for the increased length of the embryo—an increase in length, which (with the exception of the head) takes place entirely by additions at the hind end. At the stage with fourteen somites the primitive streak is still pretty long. In the later stages, as is clearly demonstrated by the tables, the diminution in the length of the primitive streak mainly concerns the posterior part and not that adjoining the embryo.
General history of the germinal layers.
The epiblast. The epiblast of the body of the embryo, though several rows of cells deep, does not become divided into two strata till late in embryonic life; so that the organs of sense formed from the epiblast, which are the same as in the types already described, are not specially formed from an inner nervous stratum. The medullary canal is closed in the samemanner as in Elasmobranchii, the Frog, etc., by the simple conversion of an open groove into a closed canal. The closure commences first of all in the region of the mid-brain, and extends rapidly backwards and more slowly forwards. It is completed in the Fowl by about the time that twelve mesoblastic somites are formed.
The mesoblast. The general changes of this layer do not exhibit any features of special interest—the division into lateral and vertebral plates, etc., being nearly the same as in the lower forms.
Illustration: Figure 105Fig. 105. Diagrammatic longitudinal section through the axis of an Embryo Bird.The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared.F.So.head-fold of the somatopleure.F.Sp.head-fold of the splanchnopleure.pp.pleuroperitoneal cavity;Am.commencing (head-) fold of the amnion;D.alimentary tract;N.C.neural canal;Ch.notochord;A.epiblast;B.mesoblast;C.hypoblast.
Fig. 105. Diagrammatic longitudinal section through the axis of an Embryo Bird.The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared.F.So.head-fold of the somatopleure.F.Sp.head-fold of the splanchnopleure.pp.pleuroperitoneal cavity;Am.commencing (head-) fold of the amnion;D.alimentary tract;N.C.neural canal;Ch.notochord;A.epiblast;B.mesoblast;C.hypoblast.
Illustration: Figure 106Fig. 106. Diagrammatic longitudinal section through the posterior end of an Embryo Bird at the time of the formation of the allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
Fig. 106. Diagrammatic longitudinal section through the posterior end of an Embryo Bird at the time of the formation of the allantois.ep.epiblast;Sp.c.spinal canal;ch.notochord;n.e.neurenteric canal;hy.hypoblast;p.a.g.postanal gut;pr.remains of primitive streak folded in on the ventral side;al.allantois;me.mesoblast;an.point where anus will be formed;p.c.perivisceral cavity;am.amnion;so.somatopleure;sp.splanchnopleure.
The hypoblast. The closure of the alimentary canal is entirely effected by a process of tucking in or folding off of the embryo from the yolk-sack. The general nature of the process is seen in the diagramsfigs.105and121. The folds by which it is effected are usually distinguished as the head-, the tail- and the lateral folds. The head-fold (fig. 105) is the first to appear; and in combination with the lateral folds gives rise to the anterior part of the mesenteron (D) (including the œsophagus, stomach and duodenum), which by its mode of formation clearly ends blindly in front. The tail-fold, in combination with the two lateral folds, gives rise to the hinder part of the alimentary tract, including the cloaca, which is a true part of the mesenteron. At the junction between the two folds there is presenta circular opening leading into the yolk-sack, which becomes gradually narrowed as development proceeds. The opening is completely closed long before the embryo is hatched. Certain peculiarities in reference to the structure of the tail-fold are caused by the formation of the allantois, and are described with the embryonic appendages. The stomodæum and proctodæum are formed by epiblastic invaginations. The communication between the stomodæum and the mesenteron is effected comparatively early (on the 4th day in the chick), while that between the proctodæum and mesenteron does not take place till very late (15th day in the chick). The proctodæum gives rise to the bursa Fabricii, as well as to the anus. Although the opening of the anus is so late in being formed, the proctodæum itself is very early apparent. Soon after the hinder part of the primitive streak becomes tucked in on the ventral side of the embryo, an invagination may be noticed where the tail of the embryo is folded off. This gradually becomes deeper, and finally comes into contact with the hypoblast at the front (primitively the apparent hind) border of the posterior section of the primitive streak. An early stage in the invagination is shewn in the diagram (fig. 106,an). It deserves to be noted that the anus lies some way in front of the blind end ofthe mesenteron, so that there is in fact a well-developed postanal section of the gut (fig. 106,p.a.g), which corresponds with that in the Ichthyopsida. For a short period, as mentioned above (p.163), a neurenteric canal is present connecting the postanal gut with the medullary tube in the duck, fowl, and other birds. On the ventral wall of the postanal gut there are at first two prominences. The posterior of these is formed of part of the tail swelling, and is therefore derived from the apparent anterior part of the primitive streak. The anterior is formed from what was originally the apparent posterior part of the primitive streak. The postanal gut becomes gradually less and less prominent, and finally atrophies.
General development of the Embryo.
It will be convenient to take the Fowl as a type for the general development of the Sauropsida.
Illustration: Figure 107Fig. 107. Dorsal view of the hardened blastoderm of a Chick with five mesoblastic somites. The medullary folds have met for part of their extent, but have not united.a.pr.anterior part of the primitive streak;p.pr.posterior part of the primitive streak.
Fig. 107. Dorsal view of the hardened blastoderm of a Chick with five mesoblastic somites. The medullary folds have met for part of their extent, but have not united.a.pr.anterior part of the primitive streak;p.pr.posterior part of the primitive streak.
The embryo occupies a fairly constant position with reference to the egg-shell. Its long axis is placed at right angles to that of the egg, and the broad end of the egg is on the left side of the embryo. The general history of the embryo has already been traced up to the formation of the first formed mesoblastic somites (fig. 107). This stage is usually reached at about the close of the first day. After this stage the embryo rapidly grows in length, and becomes, especially in front and to the sides, more and more definitely folded off from the yolk-sack.
Illustration: Figure 108Fig. 108. Embryo of the Chick between 30 and 36 hours viewed from above as an opaque object.(Chromic acid preparation.)f.b.front-brain;m.b.mid-brain;h.b.hind-brain;op.v.optic vesicle;au.p.auditory pit;o.f.vitelline vein;p.v.mesoblastic somite;m.f.line of junction of the medullary folds above the medullary canal;s.r.sinus rhomboidalis;t.tail-fold;p.r.remains of primitive groove (not satisfactorily represented);a.p.area pellucida.The line to the side betweenp.v.andm.f.represents the true length of the embryo.The fiddle-shaped outline indicates the margin of the pellucid area. The head, which reaches as far back aso.f., is distinctly marked off; but neither the somatopleuric nor splanchnopleuric folds are shewn in the figure; the latter diverge at the level ofo.f., the former considerably nearer the front, somewhere between the linesm.b.andh.b.The optic vesiclesop.v.are seen bulging out beneath the superficial epiblast. The heart lying underneath the opaque body cannot be seen. The tail-foldt.is just indicated; no distinct lateral folds are as yet visible in the region midway between head and tail. Atm.f.the line of junction between the medullary folds is still visible, being lost forwards over the cerebral vesicles, while behind may be seen the remains of the sinus rhomboidalis,s.r.
Fig. 108. Embryo of the Chick between 30 and 36 hours viewed from above as an opaque object.(Chromic acid preparation.)f.b.front-brain;m.b.mid-brain;h.b.hind-brain;op.v.optic vesicle;au.p.auditory pit;o.f.vitelline vein;p.v.mesoblastic somite;m.f.line of junction of the medullary folds above the medullary canal;s.r.sinus rhomboidalis;t.tail-fold;p.r.remains of primitive groove (not satisfactorily represented);a.p.area pellucida.The line to the side betweenp.v.andm.f.represents the true length of the embryo.The fiddle-shaped outline indicates the margin of the pellucid area. The head, which reaches as far back aso.f., is distinctly marked off; but neither the somatopleuric nor splanchnopleuric folds are shewn in the figure; the latter diverge at the level ofo.f., the former considerably nearer the front, somewhere between the linesm.b.andh.b.The optic vesiclesop.v.are seen bulging out beneath the superficial epiblast. The heart lying underneath the opaque body cannot be seen. The tail-foldt.is just indicated; no distinct lateral folds are as yet visible in the region midway between head and tail. Atm.f.the line of junction between the medullary folds is still visible, being lost forwards over the cerebral vesicles, while behind may be seen the remains of the sinus rhomboidalis,s.r.
The general appearance of the embryo between the 30th and 40th hours of incubation is shewn infig. 108from the upper surface, and infig. 109from the lower. The outlines of the embryo are far bolder than during the earlier stages.Fig. 109shews the nature of the folding, by which the embryo is constricted off from the yolk-sack. The folds are complicated by the fact that the mesoblast has already become split into two layers—a splanchnic layer adjoining the hypoblast and a somatic layer adjoining the epiblast—and that the body cavity between these two layers has already become pretty wide in the lateral parts of the body of the embryo and the area pellucida. The fold by which the embryo is constricted off from the yolk-sackis in consequence a double one, formed of two limbs or laminæ, an inner limb constituted by the splanchnopleure, and an outer limb by the somatopleure. The relation of these two limbs is shewn in the diagrammatic longitudinal section (fig. 105), and in the surface view (fig. 109) the splanchnic limb being shewn atsfand the somatic atso. Between the two limbs, and closely adjoining the splanchnopleure, is seen the heart (ht). At the stage figured the head is well marked off from the trunk, but the first separation between the two regions was effected at an earlier period, on the appearance of the foremost somite (fig. 107). Very shortly after the cephalic region is established, and before the closure of the medullary folds, the anterior part of the neural canal becomes enlarged to form the first cerebral vesicle, from which two lateral diverticula—rudiments of the optic lobes—are almost at once given off (fig. 108,op.v). By the stage figured the cephalic part of the neural canal has become distinctly differentiated into a fore- (f.b), a mid- (m.b) and a hind-brain (h.b); and the hind-brain is often subdivided into successive lobes. In the region of the hind-brain two shallow epiblastic invaginations form the rudiments of the auditory pits (au.p).