Reproduction 289 



Fig. 238. Young dogfish shortly before birth. The yolk-sac, containing a 

 remnant of the yolk of the egg, protrudes from the ventral body-wall. (Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



germ-disk has progressed so far as to produce a many-celled blastoderm 

 spreading out thin and flat on the surface of the yolk, the cells of the 

 blastoderm can be regarded as, at most, merely joint proprietors of 

 the food supply, and the yolk has become essentially extracellular. As 

 development proceeds, the blastoderm differentiates into the typical 

 germ-layers, the mesoderm splits to form somatic and visceral sheets 

 with coelomic space between them, and all of these layers progressively 

 spread over and around the nonliving yolk until eventually it is entirely 

 enclosed (Fig. 239) by splanchnopleure and somatopleure with coelom 

 between them. Thus the embryo of these large-yolked animals is put 

 to the necessity of building not only its enteron but also its body-wall 

 around its prospective food. 



In the course of development, the yolk is assimilated and utilized 

 in the building of new protoplasm. It therefore steadily decreases in 

 bulk, both relatively and absolutely. As the body of the embryo begins 

 to take form, a constriction involving both somatopleure and splanch- 

 nopleure (Fig. 239) appears between the yolk-sac and the remainder 

 of the embryo. The constriction deepens until the embryo presents the 

 appearance of a small animal having a narrow-necked globular sac sus- 

 pended from the under side of the body (Fig. 238). The amnion is 

 concerned in this constriction (Fig. 239). As the embryo increases in 

 size, the shrinking yolk-sac is drawn up into the body. The inner wall 

 (splanchnopleure) of the sac finally constitutes a small region of the 

 wall of the intestine. In elasmobranchs the somatopleure of the yolk- 

 sac finally flattens out and persists as a part of the abdominal wall. In 

 reptiles and birds, at the time of hatching, the somatopleure is ruptured 

 at the constriction between the definitive body and the extra-embryonic 

 structures and everything external to the rupture is abandoned. 



