Book - The development of the chick (1919) 4

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Lillie FR. The development of the chick. (1919) Henry Holt And Company New York, New York.

Lille 1919: Introduction | Part 1 - 1 The Egg | 2 Development Prior to Laying | 3 Outline of development, orientation, chronology | 4 From Laying to Formation of first somite | 5 Head-fold to twelve somites | 6 From twelve to thirty-six somites | Part 2 - 7 External form of embryo and embryonic membranes | 8 Nervous system | 9 Organs of special sense | 10 Alimentary tract and appendages | 11 The body-cavities, mesenteries and septum transversum | 12 Later development of the vascular system | 13 Urinogenital system | 14 Skeleton | Appendix

Part I The Early Development to the End of the Third Day

Chapter IV From Laying to the Formation of the First Somite

I. Structure of the Unincubated Blastoderm

There is more or less variation in the stage of development of iminciibated blastoderms; in exceptional cases these variations may be extreme. However, the usual condition may be described very briefly as follows (see Fig. 34): Beneath the pellucid area is the subgerminal cavity bounded marginally by the germ-wall. The posterior part only of the pellucid area is two-layered. The lower layer or gut-entoderm terminates posteriorly at the germwall, with which, however, it is not united. It is composed of spindle-shaped cells which form a coherent layer, perforated by numerous small openings that appear as breaks in the layer in section. In front of the gut-entoderm a few scattered cells appear in the subgerminal cavity. The gut entoderm does not reach the germ-wall either laterally or anteriorly, but in the course of a few hours' incubation it spreads so as to unite with the germ-wall around the entire margin of the pellucid area.

The germ-wall is slightly thicker at the posterior than at the anterior end, that is to say, that the nuclei extend deeper into the yolk (Fig. 34). There is a broad zone of junction and beyond this the margin of the blastoderm overlaps the yolk a short distance. The germ-wall has not yet become organized as a layer separate from the yolk.

The ectoderm is thicker in the region of the area pellucida than in the area opaca; and slightly thicker in the center than at the margin of the area pellucida.

The Primitive Streak

Total Views. The primitive streak is the first sign of formation of the embryo proper; it appears early on the first day of incubation as an elongated slightly opaque band occupying the posterior half or two fifths of the circular pellucid area (Fig. 35 B). It is relatively narrow in front and widens posteriorly, where it is at the same time less dense. Its anterior end usually does not quite reach the center of the pellucid area. It rapidly increases in length; the anterior end appears to be practically a fixed point, and growth takes place posteriorly probably not by addition, but between the two ends. The posterior half of the pellucid area elongates simultaneously, keeping pace with the primitive streak which lies entirely within it in the chick and most other birds. Thus the area pellucida becomes oval, then pear-shaped, and the primitive streak bisects the greater part of its length (Figs. 35, 36, 44, etc.).



Fig. 35. — Surface views of two stages of the blastoderm of the egg of the sparrow. (After Schauinsland.)

A. Before the appearance of the primitive streak.

B. The first appearance of the primitive streak.

a. o., Area opaca. a. p., Area pellucida. Ent. Th., Thickening of entoderm, pr. str., Primitive streak.


According to Koller the primitive streak takes its origin from a crescentic area at the posterior margin of the pellucid area, which he terms the sickle. The primitive streak appears as a process extending forward from the center of the sickle, and, as it grows forward, the lateral horns of the sickle are gradually taken into its posterior end. Koller's observations and interpretations have not, however, been confirmed by subsequent investigators and they would appear to rest on rather exceptional and inessential conditions.


Fig. 36. — A. Intermediate stage of the formation of the primitive streak of the sparrow. (After Schauinsland.)

B. Fully formed primitive streak of the sparrow. (After Schauinsland.)

a. o., Area opaea. a. p., Area pellucida. Ent. Th., Thickening of entoderm. Mes., Mesoderm, pr. f., Primitive fold. pr. gr., Primitive groove. pr. p., Primitive pit. pr. str., Primitive streak, s. gr., Sickle groove.

At first the surface of the primitive streak is even, but, as it elongates, a groove appears down its center. This groove is known as the primitive groove; it is bounded by the primitive folds and terminates abruptly in front in a pit, the primitive pit. which corresponds to the neurenteric canal of other vertebrates (Figs. 35, 36, 44, etc.). The primitive groove does not involve the extreme anterior end of the primitive streak, which forms a Uttle knot in front of it, the primitive knot {" Hensen's knot"). The posterior end of the primitive streak terminates in an expansion which is not very obvious in surface view, and hence is not usually described; it may be called the primitive plate (Figs. 36, 44 A, 44 B, etc). In some cases the primitive streak and groove are bifurcated at the posterior end (Fig. 44 B). The primitive streak is the first clear indication of the axis of the embryo.


The neurenteric canal is a canal that connects the posterior end of the central canal of the neural tube with the intestine. It arises from the anterior end of the primitive mouth, and is typically developed in Selachia, Amphibia, reptiles, some birds {e.g., duck, goose. Sterna, etc.). It begins in the primitive pit and extends forward into the head-process (p. 80). Subsequently the primitive pit becomes surrounded by the medullary folds, and thus opens into the neural canal. An opening is later formed through the entoderm so that the definitive canal connects neural tube and hind-gut. In the chick the neurenteric canal is never typically developed. Usually it is represented only by the primitive pit. In exceptional cases I have found traces of it in the head-process.


The so-called head-process appears in front of the primitive knot (Figs. 36 B and 44 B). In surface view it appears not unlike the primitive streak itself, but is fainter and less clearly defined. It is continuous with the primitive streak at the primitive knot, but its axis is usually a little out of line with the axis of the primitive streak.


Figs. 35 and 36 exhibit four stages of the development of the primitive streak of the sparrow (after Schauinsland). The darker area in the anterior part of the area pellucida is caused by a thicker region of the entoderm which in the course of time becomes of uniform thickness with the remainder. It will be observed that the primitive streak arises entirely within the area pellucida (Fig. 35 B). In later stages its posterior end is bifurcated (Figs. 36 A and B), and we have the appearance of a sickle somewhat similar to Roller's description for the chick. The primitive groove begins near the anterior end of the primitive streak in an especially deep pit just behind the primitive knot, and extends back the entire length of the primitive streak into the horns of the sickle. The head-process is barely indicated in Fig. 36 B.


The later history of the primitive streak is illustrated in Figs. 44, ol, 61, 65, etc.: the embryo arises in front of it around the head-process as a center; the anterior end of the primitive streak marks the hind end of the differentiated portion of the embryo. As the embryo grows in length the primitive streak decreases (cf. measurements in table), until finally, when the completion of the embryo is indicated by the formation of the tail-fold, the primitive streak disappears. The primitive knot and primitive pit occupy its anterior end at all stages, and, as the embr3"o differentiates from the anterior end of the primitive streak, the primitive pit must be regarded as moving back along the line of the primitive groove, always representing its anterior end.

Sections. The preceding sketch of the superficial appearance of the primitive streak must now be followed by a careful examination of its structure and role in the development.


Fig. 37. — Three sections through the primitive streak of a sparrow at a stage intermediate between Figs. 35 and 36. x 230. (After Schauinsland.)

A. In front of the primitive streak.

B. Through the anterior end of the primitive streak (primitive knot).

C. About through the center of the primitive streak.

All recent authors are agreed that the primitive streak owes its origin to a linear thickening of the ectoderm, from Avhich cells are proliferated between the ectoderm and the entoderm, forming a third layer, the mesoderm. Figs. 37 A, B, C show three transverse sections through a blastoderm of the sparrow slightly more advanced than the stage shown in Fig. 35 B. The first section is just in front of the primitive streak. The ectoderm is thick in the center and thins gradually toward the margin of the area pellucida, becoming decidedly thin in the region of the area opaca. The thin entoderm of the area pellucida unites peripherally with the thick yolk-sac entoderm of the area opaca. The second section passes through the anterior end of the primitive streak; the ectoderm is greatly thickened (primitive knot); the basement membrane is interrupted below, and the lowermost cells are becoming loose. The third section is through a more posterior portion of the primitive streak. The proliferation from the ectoderm is more extensive, the cells are looser and are beginning to spread out laterally. The entoderm is a continuous membrane without any connection with the primitive streak, and there are no cells between ectoderm and entoderm save those derived from the primitive streak.



Fig. 38. — Transverse sections through a very short primitive streak of the chick. Incubated 17^ hours; no head-process.

A. Through the anterior end of the primitive streak (primitive knot). Mesodermal cells are being proliferated from the ectodermal thickening; some are scattered between the two primary germ layers. The entoderm shows no proliferation, though some mesoderm cells are adhering to it.

B. Fourteen sections posterior to A. (Entire length of the primitive streak is 80 sections.) The mesoblast wings are forming; the primitive groove and primitive folds are indicated. The entoderm is free from the mesoderm.

Ect. Ectoderm. Ent., Entoderm. Mes., Mesoderm, pr. f., Primitive fold, pr.gr. Primitive groove, pr. kn., Primitive knot.


Figs. 38 A and B show the structure of the primitive streak of the chick at a more advanced stage, but before the formation of the head-process. Sections in front of the primitive streak show no cells between ectoderm and entoderm. In the region of the primitive knot (A) the ectoderm is greatly thickened, forming a projection above and below. Cells become detached from the lower surface of the ectoderm, and are converted into migratory cells between the two primary layers. Immediately behind the primitive knot the primitive groove begins abruptly; it is the seat of active proliferation from the lower layer of the ectoderm, and the cells migrate out laterally forming wings of cells, which do not, however, reach the area opaca (Fig. 38 B). Conditions are very similar along the entire length of the primitive streak at this time; but near the posterior end a few cells of the mesoderm reach the area opaca and begin to insinuate themselves between the ectoderm and the germ-wall. There is no evidence at any place that any of the mesoderm cells are derived from the entoderm. The axial thickening of the primitive groove comes in contact with the entoderm and appears in places fused to it.


Figures 39 A-E represent five sections through the head-process and primitive streak of a chick embryo at a time when the headprocess is still very short. The first section through the headprocess is described beyond. B is through the primitive knot; the ingrowth of cells is more extensive than in the preceding stage and it will be observed that they are now fused with the entoderm, so that the latter no longer appears as a distinct layer. C is through the primitive groove near its anterior end. D is a little behind the center of the primitive groove, and E is through the primitive plate. Behind the center of the primitive streak the entoderm is again free (D). It will be observed that the area of proliferation in the primitive plate is very wide.

YiG. 39. — Five sections through the head-process and primitive streak of a chick embryo. The head-process is very short.

A. Through the head-process, now fused to the entoderm.

B. Through the primitive knot.

C. Through the anterior end of the primitive groove.

D. A little behind the center of the primitive streak.

E. Through the primitive plate.

The total number of sections through the head-process and primitive streak of this series is 102. B. is 4 sections behind A. C. is 12 sections behind A. D. is 59 sections behind A. E. is 87 sections behind A.

Ect., Ectoderm. Ent., Entoderm. G. W., Germ-wall. H. Pr., Headprocess, med. pi., Medullary plate. Mes. Mesoblast. pr. f. Primitive fold, pr. gr., Primitive groove, pr. kn., Primitive knot. pr. pi., Primitive plate.


The mode of origin of the mesoderm of birds has been a very puzzling question as is proved by the numerous views that have been in vogue from time to time. One of the earhest views was that the mesoderm arose by spHtting of the primary entoderm (Remak). This view survives in part even at the present time (mesoblast of the opaque area). Balfour believed that the mesoblast in the region of the embryo "originates as two lateral plates split off from the primitive hypoblast," and that the primitive streak mesoblast is extra-embryonic, or at most enters into the formation of mesoblast of the extreme hind end of the embryo (allantois mesoblast in part). This view is found in the "Elements of Embryology" of Foster and Balfour. A third view% now of historical interest only, was that the mesoblast cells arose peripherally and migrated between the two primary germ-layers (Peremeschko, Goette). The latter author even attempted to derive the primitive streak from an aggregation of such inwandering cells. The view that the primitive streak arises as a thickening of the ectoderm and that it is the source of all the mesoderm was first stated by Kolliker, and has been accepted by Hertwdg, Rabl, and many others. It may, indeed, be regarded as definitely established for the embryonic mesoblast. Others, however, believe with His that the mesoblast of the opaque area arises by delamination from the germ-wall; this question is discussed beyond. It should also be noted that it is probable that the primitive embryonic mesoblast is supplemented in certain regions at later stages by cells proliferated from both entoderm and ectoderm, particularly in the region of the head, (gee pp. 116, 117.)

In early stages of the primitive streak the mesoblast cells are relatively sparse and bear every appearance of migrating separately. But as the ingrowth progresses and the cells become more numerous, the mesoderm becomes converted into coherent plates. These are wedge-shaped, the central broad ends fused wdth the primitive streak and the narrow margins extending laterally (Figs. 40 A, B, C). They soon overlap the margin of the opaque area and thus is produced a three-layered portion of


Fig. 40. — Three transverse sections of a late stage (corresponding to about Fio-. 44 B), through the head-process and primitive streak of a chick embryo.

A. Near the hind end of the head-process.

B. Through the primitive pit.

C. A short distance behind the center of the primitive streak. The region between the lines A-A and B-B is represented under a high magnification in Fig. 41.

Bl. I., Blood island, coel. Mes., Coelomic mesoblast. Ect., Ectoderm. Ent., Entoderm. G. W., Germ-wall. med. pi., Medullary plate. Mes., Mesoderm. N'ch., Notoehord. pr. f., Primitive fold. pr. gr., Primitive groove, pr. p.. Primitive pit.


the latter which corresponds to the future vascular area. The mesoblast grows out, not only from the sides of the head-process and primitive streak, but also from the hind end of the latter, that is from the primitive plate. The mesoblast thus extends into the opaque area behind the embryo at a very early stage (Figs. 42 and 44).

The primitive groove must be regarded as an expression of the forces of invagination of the mesoblast, and the primitive folds as the lips of this invagination.


Fig. 41. — The part of the section shown in Fig. 40 C, between A-A and B-B more highly magnified. Abbreviations same as Fig. 40.

The Head-process

Two stages of the head-process are shown in tranverse section a short distance in front of the primitive knot in Figs. 39 A and 40 A. It consists of a thicker central mass of cells with lateral wings; the central part, or primordium of the notochord, is continuous posteriorly with the axis of the primitive streak. These two portions of the mesoblast are often termed gastral and prostomial, connected with the head-process and primitive streak respectively. The head-process becomes inseparably fused with the entoderm in the middle line immediately after its formation; and this fusion is continued back along the axis of the primitive streak (Figs. 39 and 40). The fusion is particularly intimate and persistent at the extreme anterior end of the head-process; behind this point the notochord and entoderm soon separate again in the course of development. But the anierior end of the notochord remains attached to the entoderm for a considerable period after the formation of the headfold. A longitudinal section shows the head-process as an appendage to the anterior end of the primitive streak, or the primitive knot (Fig. 42).


Fig. 43. — Diagrams to illustrate the theory of concrescence as applied to the primitive streak of the bird. The central area bounded by the broken line represents the pellucid area ; external to this is the area opaca, showing as concentric zones the germ-wall (G. W.), the zone of junction (Z. J.), and the margin of overgrowth (M. O.). m. n., Marginal notch. For description see text.


The most obvious interpretation of the head-process is as an outgrowth from the primitive knot. But another, and more probable interpretation in view of all the facts, is that the headprocess is a later stage of the anterior end of the primitive streak; that a gradual separation of the ectoderm takes place in the axis of the primitive streak beginning at the anterior end, and progresses posteriorly. That part in which the ectoderm is separated represents the head-process; it has therefore the same composition as the primitive streak, except that the ectoderm has become independent.


Interpretation of the Primitive Streak. The discussion of the significance of the primitive streak involves two parts: (1) its morphological significance, and (2) its role in the formation of the embryo. The first question involves knowledge of comparative embryology, which is not assumed for the purposes of this book, and it will therefore be considered very briefly. The fundamental relations of the primitive streak must define its morphological interpretation; the first thing to be noted is that the germ-layers, more especially the ectoderm and mesoderm, are fused in the primitive streak; second, the differentiated part of the embryo is formed in front of it; third, the neurenteric canal occupies the anterior end of the primitive streak; fourth, the anus forms at its posterior end. Now these characters are exactly those of the blastopore or primitive mouth of lower vertebrates, that is of the aperture of invagination of the archenteron. For these reasons, and because in all other essential respects the primitive streak corresponds to the blastopore, it must be interpreted as the homologue of the latter. It is to be regarded, therefore, as an elongated blastopore, and the primitive groove as a rudimentary archenteric invagination.


This interpretation raises the question as to its relation to the original marginal area of invagination of the entoderm. Can these two things be really different stages of the same thing? The concrescence theory gives a theoretical basis for their identification. It will be remembered that the margin of invagination represents a small section of the margin of the primitive blastoderm in the pigeon, and, by inference, in the chick also. The remainder of the margin where the zone of junction persists is the margin of overgrowth. Now we assume that the closure of the original marginal area of invagination proceeds by concrescence or coalescence of its lips, beginning in the middle line behind, thus producing a suture which is the beginning of the primitive streak. Let the above circles (Fig. 43) represent the blastoderm in four stages of closure of the original area of invagillation. The shaded margin represents the zone of junction, the unshaded portion of the margin represents the area of invagination of the entoderm. The dotted contour represents the margin of the pellucid area. In A the middle of the area of invagination is marked 1, and corresponding points to the right and left 2, 3, and 4. In diagram B it is supposed that the margin of invagination is turned forward at 1, and that the lateral portions are brought together as far as 2, thus producing a suture in the middle line 1-2 continuous with the margin 3-4. The zone of invagination is correspondingly reduced in extent and the zone of junction increased. In diagram C the lateral lips of the zone of invagination are represented as completely concresced, thus producing a median suture 1, 2, 3, 4, extending through the posterior half of the area pellucida to the margin. The zone of junction is on the point of closing behind the line of concrescence which is the primordium of the primitive streak. In diagram D, finally, the opaque area has closed in behind the line of concrescence which occupies the hinder half of the pellucid area.


To apply this theory to the actual data of the development, it is only necessary to assume that the entoderm separates from the ectoderm along the line of concrescence, and that the primitive streak arises subsequently along the same line. The actual demonstration of the truth of this conception cannot be furnished bv observation alone, however detailed. It is, however, possilDle to test it by experiment, though difficult because the concrescence must take place, if at all, prior to laying. The strong support of the theory lies at present in the data of comparative embryology; in the lower vertebrates the mesoderm and entoderm are both formed from the margin of invagination.


Summarizing the matter, we may say that in the chick gastrulation is divided into two separate processes: the first is the invagination of the entoderm from the margin, and the second is the ingrowth (or invagination) of mesoblast and notochord from the primitive streak, which represents the coalesced lips of the margin of invagination; the primitive groove is therefore the expression of a second phase of invagination.


The genetic relation of the primitive streak to the margin of the blastoderm is well illustrated by an abnormal blastoderm described bv Whitman in which the primitive groove was continned across the area opaca to a marginal notch at the posterior end. A similar marginal notch at the hinder end of the blastoderm in the line of prolongation of the primitive streak has been described also by His and Raiiber, but in the cases observed by them there was no connection with the primitive groove. It suggested to them, however, the idea of genetic connection between the two, and was used as argument for the derivation of the primitive streak from the margin by concrescence.


The second question concerning the primitive streak, its role in the formation of the embryo, may be answered very briefly by saying that it is itself the primordium of the greater portion of the axis of the embryo; some indeed maintain that it represents the entire embryonic axis excepting the short pre-chordal part (Kopsch). The view of Balfour and Dursy that it takes no essential part in the formation of the embryo, but atrophies as the embryo forms, is now of historical interest only. The question is how much of the embryo is represented by the primitive streak. But this question is by no means easy to answer, and there is no complete agreement in regard to it. The one point that is definitely settled is that the anus arises at the hinder end of the primitive streak; but what point in the embryo corresponds to the anterior end of the primitive streak, or, in other words, how much of the embryo is laid down in the blastoderm in front of the primitive streak, is a disputed question. The attempt has been made to solve the problem by destroying the anterior end of the primitive streak by a hot needle, or by electrolysis, then sealing up the egg and permitting it to develop farther and finally locating the resultant injury in the embryo. But, while one worker finds the injury at the anterior end of the notochord (Kopsch), that is in the region of the fore-brain, another finds it in the region of the heart, that is in the hind-brain (Peebles). The reasons for this discrepancy in results are two: (1) the methods employed are not sufficiently exact, and (2) it is difficult in the living egg to determine the exact location of the anterior end of the primitive streak, and sometimes even to distinguish it from the head-process. Owing to the extremely rapid growth of all parts of the embryonic axis, a minute division of the primitive streak becomes a relatively long part of the embryonic axis in a very short time. It is obvious, therefore, that the slightest deviation of the injury from the point aimed at may lead to considerable error in the results. The result of Kopsch, however, is more consistent with our knowledge of other forms.

III. The Mesoderm of the Opaque Area

We have seen that the mesoderm arises from the sides of the head-process and the primitive streak, and grows out between the ectoderm and the entoderm to the margin of the pellucid area; it then begins to overlap the opaque area at first behind, later at the sides, appearing between the ectoderm and the germwall. Figs. 44 A, B, C, and 45 illustrate its peripheral extension; at first it spreads most rapidly behind the embryo, but soon extends with equal speed opposite the primitive streak, and thus a considerable portion of the area opaca becomes three-laj^ered, consisting of ectoderm, mesoderm, and germ-wall (Figs. 40 C and 41). The contour of the anterior margin of the mesoderm it as first rounded, convex anteriorly (Figs. 44 A and B). Then the antero-lateral angles of the mesoblast begin to extend forward so that the anterior boundary becomes concave (Fig. 44 C) ; the lateral horns thus established continue to grow forward and ultimately meet in front of the head (Fig. 45) ; they thus bound a mesoblast-free area in front of and beneath the head, known as the proamnion, into which the mesoderm does not penetrate until a relatively late stage of development.

Blood-islands (Figs. 44 C and 45) develop early in the threelayered part of the opaque area; appearing first behind the embryo, they rapidly differentiate forward opposite the sides of the embryo and follow the expansion of the mesoblast. This three-layered portion of the opaque area is known as the vascular area (area vasculosa) after the appearance of the blood-islands. It soon acquires a very definite peripheral boundary by the formation of the vena (sinus) terminalis at its margin (Fig. 45). The two-layered peripheral portion of the opaque area is known as the vitelline area (area vitellina), and here again we distinguish two zones, an outer including the zone of junction, and an inner one (Figs. 32, 33).

The first blood-islands are masses of cells lying on the germwall behind the embryo; the first blood-cells (erythrocytes) and blood-vessels arise from them, hence their name. Soon after their origin the blood-islands appear red owing to the formation of haemoglobin. Between the blood-islands and the ectoderm is a layer of the mesoderm (Fig. 41). If the blood-islands be reckoned as mesoderm we must distinguish two layers of the latter, viz., a deep or vascular layer (angioblast) lying next the germ-wall, and an upper layer next the ectoderm, which may be called the ccelomic mesoderm, inasmuch as the body-cavity (coelome) develops within it later.


Fig. 45. — Blastoderm and embryo at the stage of fourteen somites. The horns of mesoblast are on the point of meeting in front of the head.

a. p., Area pelkicida. a. vase, Area vaseulosa. a. v. i.. Area viteUina interna. Ht., Heart, n. F., Neural folds, pr'a., Proamnion, pr. str., Primitive streak. S. t., Sinus terminalis.


There are two sharply contrasted views concerning the origin of the mesoblast in the area opaca. According to the one point of view it is simply a peripheral extension of the primitive streak mesoblast with which as a matter of fact it is continuous (Hertwig, Rabl, and others). According to the other point of view it is split off from the germ-wall (His and others). One thing is perfectly clear, viz., that the mesoderm of the opaque area arises in continuity with the primitive streak mesoderm; the second view would therefore be better expressed, as Riickert states it, that the primitive streak mesoderm grows in the region of the area opaca at the expense of elements of the germinal wall.

If the cells of the primitive streak mesoblast be compared with the cells of the forming blood-islands a sharp contrast is observed; the mesoblast cells of the area pellucida are devoid of yolk-granules; young blood-islands on the other hand contain yolk-granules of precisely the same character as those of the germ-wall (Fig. 41), which must have been derived from the latter. If the origin of the blood-islands be carefully traced, they are found to be rooted in the protoplasm of the germ-wall; and prior to the appearance of the blood-islands proper, protoplasm and nuclei of the germ-wall aggregate superficially in a manner that appears to foreshadow the blood-islands. Therefore, either the blood-islands are derived from the cells of the germ-wall, or cells of the mesoderm growing over the germ-wall burrow into the latter, engulf yolk-spheres, and reappear in masses as bloodislands. Patterson (1909) has shown by an experimental study that in any region in which primitive streak mesoblast is prevented from reaching the germ- wall, blood-islands fail to develop. The second alternative is therefore probably right in principle.


Another question concerns the origin of the layer of coelomic mesoblast that overlies the blood-islands: is it derived from the primitive streak mesoblast, or is it split off from the blood-islands? When the latter first appear, in the periphery of the vascular area at least, there is no coelomic mesoblast above them. It appears later, at first not as a coherent layer, but as scattered cells that rapidly unite to form a layer. In many places the microscopical appearances indicate strongly that the cells are split off from the surface of the blood-islands; but, as they are usually not far from the edge of the advancing coelomic mesoblast, it may be that they are derived from the latter. Riickert states, however, that, in the case of some isolated blood-islands behind the embryo, a layer of mesoblast is formed over them while they are still isolated. This would render the derivation from the blood-islands probable in such cases. It is possible, therefore, that the coelomic mesoblast grows partly, at least, at the expense of the superficial cells of blood-islands.


As rapidly as they are formed the various blood-islands connect and anastomose with one another, forming a vascular network Ivino; between the coelomic mesoblast and the remains of the germ-wall. This network spreads throughout the vascular area, and appears later in the pellucid area, and communicates with the blood-vessels of the embryo (Figs. 44 and 45). In the next chapter we shall consider the manner in which the extension takes place, and the origin of the blood-vessels and blood-cells.

IV. The Germ-wall

The germ-wall arises, as we have seen, through infiltration of the superficial white yolk by the periblast. These cells multiply and anastomose and form a multinucleated syncytium with the yolk-granules in its meshes. By degrees the protoplasm itself takes up the j^olk-granules, which are gradually digested, and the germ-wall thus becomes organized as a coherent layer. It then separates from the underlying yolk. The next period in the history of the germ-wall is its differentiation, which takes place in the vascular area concomitantly Avith the formation of the bloodislands: a considerable proportion of the protoplasm and nuclei of the germ-wall accumulates at the surface and forms the vascular mesoderm in the manner already described. The part of the germ-wall that remains after the separation of the mesoderm then differentiates into the characteristic entodermal epithelium of the opaque area, which is known as the yolk-sac epithelium (entoderm) because it is destined to form the lining of the yolk-sac.

After the formation of the vascular area the term germ-wall must be restricted to the lower layer of the vitelline area, because within the vascular area it has already differentiated into the mesoderm and yolk-sac entoderm. The development of the germ-wall takes place in a centripetal direction; at any period during the overgrowth of the yolk the three stages of the germwall may be found in the concentric zones. The first stage, that of periblast, is found in the zone of junction (area vitellina externa); the second stage, that of organization of the germwall, is found in the area vitellina interna; and the third stage, that of differentiation, is found at the margin of the area vasculosa. Within the latter area the differentiation is completed.



Lille 1919: Introduction | Part 1 - 1 The Egg | 2 Development Prior to Laying | 3 Outline of development, orientation, chronology | 4 From Laying to Formation of first somite | 5 Head-fold to twelve somites | 6 From twelve to thirty-six somites | Part 2 - 7 External form of embryo and embryonic membranes | 8 Nervous system | 9 Organs of special sense | 10 Alimentary tract and appendages | 11 The body-cavities, mesenteries and septum transversum | 12 Later development of the vascular system | 13 Urinogenital system | 14 Skeleton | Appendix

Cite this page: Hill, M.A. (2019, October 14) Embryology Book - The development of the chick (1919) 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_development_of_the_chick_(1919)_4

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G