Book - The Early Embryology of the Chick 6

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Chapter 6 From the Primitive Streak Stage to the Appearance of the Somites

The Primitive Streak as a Center of Growth

The importance of the primitive streak embryologically , is due chiefly to the way it is involved in the establishment of the germ layers. Representing as it does the fused lips of the blastopore it marks the location of entoderm invagination. The mesoderm also arises at the primitive streak region. The general appearance and the location of the primitive streak are both well shown in embryos of i6 hours of incubation (Fig. 8). In embryos which have been incubated i8 hours (Fig. ii) the primitive streak is still the most conspicuous feature. Structurally it is Uttle changed from the conditions seen in i6-hour chicks, but it appears to be somewhat more caudally located. In 21 to 22-hour embryos (Fig. 14) the primitive streak hes still farther caudal in the blastoderm. /Its change in position is relative rather than actual. The apparent change in the position of the primitive streak is due to the fact that.growth is taking place more rapidly cephalic to it than caudal to it. This tendency is in evidence throughout the early growth of the embryo. The cephalic region is precocious in development. As development progresses we shall find the primitive streak occupying a constantly more posterior position in the body and being more and more overshadowed by the greater growth of the structures lying cephalic to it.

The structure of the primitive streak region is best shown by transverse sections. In the sections diagrammed in Figure 13, a different conventional scheme of representation has been Employed to indicate each of the germ layers. The ectoderm is vertically hatched, the cells of the mesoderm are represented by heavy angular dots when they are isolated or by solid black lines when they lie arranged in the form of compact layers, and the entoderm is represented by fine stippling backed by a single line. This same conventional representation of the different germ layers is observed in all diagrams of sections in order to facilitate following the way in which the organ systems of the embryo are constructed from the germ layers. Details of cell structure are for the most part omitted with the expectation that the student will acquire, a knowledge of them in his own study of sections. The plane in which each of the sections diagrammed passes through the embryo is indicated by a line drawn on a small outline sketch of an embryo of corresponding stage. For interpretation these outhne sketches should be compared with actual specimens or detailed drawings of entire embryos of the same stage of development.


Fig. 11. Dorsal view ( X 14) of entire chick embryo of 18 hours incubation.


In embryos of the stage under consideration the relationship of the germ layers at the primitive streak still indicates their manner of derivation (Fig. 13, C and D). The ectoderm and the entoderm are continuous with each other without any demarcation. The mesoderm arises from the primitive streak where ectoderm and entoderm merge and grows laterad on both sides of the primitive streak extending into the space between ectoderm and entoderm. The mass of cells in the floor of the primitive groove is to be regarded as constituting an undifferentiated area from which new cells are being proliferated rapidly and are emigrating to become components of one or another of the germ layers:

To those who have studied the embryology of more primitive vertebrates, particularly the Amphibia, the fact that the lips of the blastopore constitute centers of growth from which cells are pushed forth to take part in the formation of the differentiated germ layers will already be familiar. The fact that the blastopore of the chick has suffered a change in position due to concrescence, and has in the same process become closed by fusion of its lips must not be allowed to obscure its homologies. In attempting to bring the relationships of the germ layers in the chick into line with the relationships of the germ layers in embryos having less yolk, it will be of great assistance to picture a chick Kfted off the yolk and the lateral margins of the blasto derm pulled together ventrally. Or the method of comparison may be reversed if one imagines the embryo of a form' having less yolk, such as an amphibian, to be split open along the midventral line and spread out on the surface of a sphere as a chick lies on the yolk.

In Figure 13, D, a small region at the primitive streak has been drawn at higher magnification to show the characteristic cellular structure of the undifferentiated region in the floor of the primitive groove and of the various layers merging at this place. The cells of the ectoderm are much more closely packed together and more sharply dehmited than those of the other germ layers. Where the ectoderm is thickened in the primitive ridge region, it is several cell layers thick (stratified). ¬´ (Fig. 13, D.) In regions lateral to the primitive ridge it gradually becomes thinner until it consists of but a single cell layer (Fig. 13, jE). The rapid extension that the mesoderm is at this time undergoing is indicated by the loose arrangement and sprawling appearance of its cells. Their irregular cytoplasmic processes make them look much like amoebae fixed during locomotion.

The cells of the entoderm are neither as closely packed nor as clearly defined as are the ectoderm cells. Nevertheless, in contrast to the condition of the mesoderm at this stage, the entoderm cells form a definite, unbroken layer.

The Growth of the Entoderm and the Establishment of Primitive Gut

Sections of embryos of this stage show how the entoderm has spread out and become organized into a coherent layer of cells merging peripherally with the inner margin of the germ wall and overlapping it to a certain extent (Fig. 13, C, £, F)s The cavity between the yolk and the entoderm which has been called the gastroccele is now termed the primitive gut. The yolk floor of the primitive gut does not •show in sections prepared by the usual methods. The reasons for this are to be found in the relations of the embryo to the ' -^ yolk A^efore it is removed for sectioning^ In the entire central region of the blastoderm lie yolk is separated from the entoderm by the cavity of the primitive gut. When the embryo is removed from the yolk sphere the yolk floor of the primitive gut, not being adherent to the blastoderm, is left behind. In contrast the peripheral part of the blastoderm Ues closely applied to the yolk. Some yolk adheres to this part of the blastoderm when it is removed. This adherent yolk is shown in the section diagrams of Figure 13. Its presence clearly indicates why this region (area opaca) appears less translucent in surface views of entire embryos.

In embryos of 18 hours the primitive gut is a cavity with , a flat roof of entoderm and a floor of yolk. Peripherally it is bounded on all sides by the germ wall (Fig. 13, C, F). The merging of the cells of the entoderm with the yolk mass is shown in the small area of the germ wall drawn to a high magnification in Figure 13, E. In the germ wall cell boundaries are incomplete and very difficult to distinguish but nuclei can be made out surrounded by more or less definite areas of cytoplasm. This cytoplasm contains numerous yolk granules in various stages of absorption. It will be recalled that the nuclei of the germ wall arise by division from the nuclei of cells lying at the margins of the expanding blastoderm. They appear to be concerned in breaking up the yolk in advance of the entoderm as it is spreading about the yolk sphere. About the twenty-second hour of incubation indications can be seen of a local differentiation of that region of the primitive gut which underlies the anterior part of the embryo. By focusing through the ectoderm in the anterior region of a wholemount of this age a pocket of entoderm can be seen (Fig. 14). This entodermal pocket is the first part of the gut to acquire a floor, other than the yolk floor, and is called from its anterior position the fore-gut. Consideration of the fore-gut except to note the location of its first appearance can advantageously be defei;red because its origin and relationships are more readilv appreciated from the study of somewhat older embryos.

The Growth and Differentiation of the Mesoderm

The mesoderm which arises either side of the primitive streak spreads rapidly laterad and at the same time each lateral wing of the mesoderm swings cephalad. Figure 12 shows schematically the extension of the mesoderm during the latter part of the first day of incubation. The diagonal hatching represents the mesoderm seen through the transparent ectoderm. The principal landmarks of the embryos are sketchily represented.

It will be noticed that the manner in which the mesoderm spreads out leaves a mesoderm-free area in the anterior portion 41 of the blastoderm. This region is known as the proamnion. The name might carry the inference that this area is the primordium of the amnion, a structure which first appears near this region somewhat later in development. Such is not the fact. The term proamnion was applied to this region before its true / significance was understood. It is not the precourser of the ^ amnion. In dorsal views of entire embryos the proamnion is readily located by reason of its lesser density. The proamnion is bounded anteriorly by the area opaca, posteriorly in the midline by the thickened anterior part of the embryo, and posteriorly on either side by the anterior border of the mesoderm (Fig. 12). The importance of the proamnion lies chiefly in the indication it gives of the progress of mesoderm extension. The rapid growth that the mesoderm of the anterior region is undergoing at this stage is clearly indicated by the diminution in area of the proamnion in embryos of 22 hours as compared with embryos of 18 hours (Fig. 12).

Sections passing through the primitive streak of embryos of this stage show the pair of loosely aggregated masses of mesoderm extending to either side between the ectoderm and entoderm. As would be expected from the method of origin, little mesoderm appears in the mid-line except posterior to the primitive streak. Immediately to either side of the mid-hne the mesoderm is markedly thicker than it is farther laterad (Fig. 13, B). In whole-mounts the positions of the regional thickenings of the mesoderm are evidenced by the greater opacity they impart to the embryo locally (Fig. 14). These thickened zones of the mesoderm are the primordia of the dorsal mesodermic plates. Because of the way in which they are later divided into metamerically arranged cell masses or somites they are frequently designated as the segmental zones of the mesoderm. The segmental zones are in early stages most clearly marked somewhat cephalic to Hensen's node, where the first somites will appear. As they extend caudad on either side of the primitive streak they gradually become less and less definite.


F1G.12. Schematic diagrams to show the extension of the mesoderm during the latter part of the first day of incubation. Some of the more prominent structural features of the embryos are drawn in lightly for orientation but the ectoderm is supposed to be nearly transparent allowing the mesoderm to show through. The areas into which the mesoderm has grown are indicated by diagonal hatching.



Fig. 13. Sections of 18-hour chick. The location of each section is indicated by a line drawn on a small outline sketch of an entire embryo of corresponding age. The letters affixied to the lines indicating the location of the sections correspond with the letters designating the section diagrams. Each germ layer is represented by a different conventional scheme: ectoderm by vertical hatching; entoderm by fine stippling backed by a single line; and the cells of the mesoderm which at this stage do not form a coherent layer, by heavy angular dots.

A, diagram of transverse section through notochord; B, diagram of transverse section through primitive pit ; C, diagram of transverse section through primitive streak; D, drawing showing cellular structure in primitive streak region; E, drawing showing cellular structure at inner margin of germ wall; F, diagram of median longitudinal section passing through notochord and primitive streak.


The sheet-like layers of mesoderm which are characteristic of the mid-body region do not extend to the anterior part of the embryo. The mesoderm of the future head region is derived from mesoderm cells which invade the head from the more definitely organized layers of mesoderm lying posterior to it. The cephalic mesoderm for this reason never shows the regional differentiations and the organization into definite layers which later appear in the mesoderm of the mid-body region.

The Formation of theNotochord

The notochord arises in the chick as a median out-growth from the rapidly proliferating, undifferentiated cells at the cephalic end of the primitive streak (Fig. 13, F). The way in which the notochord grows cephalad from the anterior end of the primitive streak, just as in other vertebrate embryos it arises from the region of the anterior lip of the blastopore, is one of the points which confirms the identification of the primitive streak of the chick as the closed blastopore. Largely because of the way in which the notochord arises in Amphioxus, a primitive vertebrate of doubtful relationships, it has usually been considered of entodermal origin. In Amphibia /and in birds it arises not from any definite germ layer but from the undifferentiated growth center about the blastopore which is giving rise to both entoderm and mesoderm. Even in Amphioxus the notochord arises at the same time and in the same manner as the mesoderm. In its later differentiation the notochord resembles mesodermal derivatives more closely than entodermal. The common origin of notochord and mesoderm, and the unmistakably mesoderal characteristics of the fully developed notochord should be emphasized rather than the early association of the notochordal primordium with the entoderm and its doubtful origin therefrom. For these reasons the notochord is in this book treated as a mesodermal structure. In entire embryos of 18 to 22 hours (Figs. 11 and 14) the notochord can be seen in the mid-line extending cephalad from Hensen's node. Hensen's node is at once the posterior Umit of the notochord and the anterior end of the primitive streak. The notochord and the primitive streak together clearly mark the mid-line of the embryo and establish definitely the longitudinal axis of the developing body. In sections (Fi^. 13, A, F) the notochord is not at this early stage sharply difterentiated from the loosely arranged mesoderm cells adjacent to it. In later stages, however, the cells composing it becomes aggregated to form a characteristic rod-shaped structure, circular in cross section and with clearly defined boundaries (Fig. 52, C).

The Formation of the Neural Plate

In surface views of entire chicks of about 18 hours (Fig. 11) areas of greater density may be made out on either side of the notochord. These areas extend somewhat anterior to the cephaUc end of the notochord where they appear to blend with each other in the mid-line. Sections of this region (Fig. 13, A) show that the greater density seen in whole-mounts is due to thickening of the ectoderm. Rapid cell proliferation has resulted in the ectodeim in the middle region becoming several cells in thickness. This thickened area is known as the neural (medullary) plate. Laterally the thickened ectoderm of the neural plate blends without abrupt transition into the thinner ectoderm of the general blastodermic surface. Anteriorly the neural plate is more clearly marked than it is posteriorly. At the level of Hensen's node the neural plate diverges into two elongated areas of thickening one on either side of the primitive streak

++++++++++++++++++ Fig. 14. Dorsal view ( X 14) of entire chick embryo of about 21 hours incubation.

In embryos of 21 or 22 hours (Fig. 14) the neural plate becomes longitudinally folded to estabhsh a trough known as the neural groove. The bottom of the neural groove lies in •1 the mid-dorsal line. Flanking the neural groove on each side is a longitudinal ridge-like elevation involving the lateral portion of the neural plate. These two elevations which bound the neural groove laterally are known as. the neural folds. The folding of the originally flat neural plate to form a gutter, flanked on either side by parallel ridges, is an expression of the same extremely rapid cell proliferation which first manifested r itself in the local thickening of the ectoderm to form the neural plate. The iormation of the neural plate and its subsequent folding to form the neural groove are the first indications of^the differentiation of the central nerv ous system.

The Differentiation of the Embryonal Area

Due to the thickening of the ectoderm to form the neural plate and also to the thickening of the dorsal zones of the mesoderm, the part of the blastoderm immediately surrounding the primitive streak and notochord has become noticeably more dense than that it is in the peripheral portion of the area pellucida. Because it is the region in which the embryo itself is developed this denser region is known as the embryonal area. Although the embryonal area is at this early stage directly continuous with the peripheral part of the blastoderm without any definite Une of demarcation, they later become folded off from each other. The peripheral portion of the blastoderm is then spoken of as extra-embryonic because it gives rise to structures which are not built into the body of the embryo, although they play a vital part in its nutrition and protection during development.

The anterior region of the embryonal area is thickened and protrudes above the general surface of the surrounding blastoderm as a rounded elevation. This prominence marks the region in which the head of the embryo will develop (Fig. 14). The crescentic fold which bounds it is termed the head fold and is the first definite boundary of the body of the embryo. % JdJ^ Throughout the course of development we shall find the head region farther advanced in differentiation than other parts of the body. This is a repetition of race history in the development of the individual, for phylogenetically the head is the oldest and most highly differentiated region of the body. It is one of many manifestations of the law of recapitulation, in conformity with which the individual in its development rapidly repeats the main steps in the development of the race to which it belongs.