from all directions. The cells move to this depression or 

 notch, then downward, and finally outward on the under- 

 side of the blastodisc. 



The blastodisc expands in all directions except at the 

 blastopore margin. This overgrowth is a continuing process 

 and results in the blastopore being converted from a trans- 

 verse line to a V-shape and finally a midline groove, as the 

 margins of overgrowth meet behind it. The blastopore is 

 now identified as the primitive streak. Marginal growth 

 continues behind the primitive streak, and the latter also 

 moves posteriorly, laying down the axis of the embryo. The 

 primitive streak has at its anterior end a Hensen's or primi- 

 tive node, behind which is the primitive pit. Concrescence 

 continues through all of this period with cells moving back 

 and medially to the primitive streak, down through the 

 streak, and then forward and laterally. Involution and in- 

 gression are particularly marked at the primitive pit, which 

 corresponds to the dorsal lip of the blastopore. From this 

 pit, the notochord extends forward along the midline be- 

 tween plates of mesoderm. 



Entoderm and mesoderm separate as they move out from 

 the primitive pit and streak. The entoderm forms a com- 

 plete sheet below the blastodisc, while the mesoderm grows 

 out and around an area in front of the head. The spread of 

 the mesoderm is marked by the appearance of the blood 

 islets and eventually the sinus terminalis. Within the body, 

 the mesoderm forms segmental blocks, the somites, while 

 laterally it is a continuous plate which later splits into two 

 layers to form a coelom, both within the embryo and out- 

 side the embryo. 



As a result of the gastrulation process, the blastocoel is 

 converted into the archenteron, a process quite different 

 from that heretofore observed. 



The anterior end of the embryo, which is approximately 

 in the position of the original blastopore, is well advanced 

 in its development and differentiation as compared with the 

 posterior part of the blastodisc, which is still expanding 

 marginally, still rolling in along the primitive groove, and 

 differentiating entoderm, mesoderm, and notochord. The 

 anterior end of the embryo with its neural ridges and groove 

 is gradually raised up from the sheet. As this head fold sep- 

 arates from the blastoderm, it extends forward both by 

 growth and by posterior constriction of the area of attach- 

 ment to the yolk sac. The underlying entoderm is also 

 effected in the separation of the head fold and produces a 

 pocket, the foregut. The separation of the embryo from the 

 blastodisc continues from front to back until much of the 

 anterior half of the body is formed. About this time, a tail 

 fold appears, rises up, and a part of the entoderm is cut off 

 within it as the hindgut. The head and tail portions con- 

 strict from the blastodisc until only a stalk connects the 

 embryo with the yolk sac. 



The embryo, as it increases in size and the head flexes to 

 the right, begins to sink down into the surface of the disc. 

 As it sinks, the surface membranes form a fold which moves 



back over the head region as that grows in size. A similar 

 fold appears over the tail part, and eventually the folds meet 

 above the embryo to enclose it in an amniotic cavity. This 

 cavity is lined by a layer of ectoderm called the amnion. 

 The amnion is covered on its internal surface by a layer of 

 mesoderm which separates it from the extraembryonic 

 (outside the embryo) coelom. The outer cover of these 

 extraembryonic tissues is the chorion (see Figure 9-6). The 

 chorion, a layer of ectodermal cells, along with its meso- 

 derm is called the serosa. 



As the embryo lifts from the blastodisc and the yolk-sac 

 stalk constricts, a diverticulum of the hindgut, the allan- 

 tois, appears and grows rapidly. This extends out through 

 the umbilicus and expands in the extraembryonic coelom 

 as a storage vesicle for waste products from the excretory 

 system. Meanwhile, the blastodisc has been extending 

 around the yolk to enclose it as a yolk sac. There is, between 

 the entoderm and yolk, a layer of periblast cells. The peri- 

 blast in the chick is never well marked and appears to con- 

 tribute new cells to the overlying entoderm. At hatching, 

 when the remaining yolk lies free in the gut, it can be as- 

 sumed that some periblast cells still adhere to it. Thus, in 

 the chick a part of the entoderm is lost and a part of the 

 yolk never becomes intracellular. 



Since telolecithal eggs show a wide range of variation in 

 their development, some of these modifications should be 

 examined. 



Shark The egg of the shark is strongly telolecithal and 

 undergoes discoidal cleavage much as that of the chick 

 (Figure 7-10). The early cleavages separate a blastodisc 

 from a layer of periblast cells. These cells form a syncytium 

 (cells not separate from each other by membranes) over the 

 yolk below and at the margin of the blastodisc. A seg- 

 mentation cavity, called the blastocoel, forms between the 

 central part of the periblast and the blastodisc. As it 

 gradually expands, the blastodisc becomes many cells 

 thick, and the marginal growth is primarily directed poste- 

 riorly. The posterior margin and lateral margins tend to 

 rise up slightly from the yolk mass as the blastoderm spreads, 

 and this edge turns under to form a two-layered germ ring. 

 At the posterior margin, the germ ring is raised to form a 

 blastopore, and the separation of overlying epiblast and in- 

 voluted hypoblast is more marked. 



Gastrulation begins with the formation of a dorsal blasto- 

 pore lip which gradually condenses to a primitive pit. Gas- 

 trulation involves concrescence and involution particularly 

 of mesoderm; the entoderm spreads on the underside of the 

 hypoblast mainly by cellular proliferation. As gastrulation 

 continues, the blastodisc becomes two-layered. The blastocoel 

 now lies between the outer ectoderm and the inner meso- 

 derm. 



The earliest invaginated mesoderm produces a prechordal 

 plate. This has a notochordal section in the middle of the 

 roof, and bilateral mesodermal plates joining the entoderm 



CLEAVAGE, GASTRULATION, ORGANOGENESIS • 203 



