112 



COMPARATIVE ANATOMY 



amount of yolk is contained within cells which, although more or less 

 encumbered by the yolk, are yet capable of developmental activity. 

 Gastrulation having established the enteron, the greater part of the 

 embryonic food is then present, not in the enteric cavity but, even better 

 than that, within the cells which constitute the wall of the enteron where 

 it may be directly acted upon by the endodermal protoplasm and made 

 available, as the blood system develops, for transportation to all parts of 

 the growing embryo. 



The enormous yolk of the egg of a reptile or bird is morphologically 

 a part of the original ovum. But by the time cleavage of the germ disc 

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

 out thin and flat on the surface of the volk, the cells of the blastoderm can 



Fig. 83. 



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

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



be regarded as, at most, merely joint proprietors of the food supply and the 

 yolk is essentially extra-cellular. As development proceeds, the blasto- 

 derm 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 non-living 

 yolk (Figs. 86 and 87) until eventually it is entirely enclosed by splanch- 

 nopleure and somatopleure with coelom between them. 



In the amphibian the food supply is, from the beginning, not only 

 inside the embryo but, for the most part, inside the prospective wall of the 

 enteron. In reptile and bird the food is external to the early embryo. 

 Therefore the embryo is put to the necessity of building not only its 

 enteron but its body wall around its prospective food. 



As development proceeds 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 splanchnopleure, 

 appears between the yolk-sac and the remainder of the embryo (Fig. 82). 

 The constriction deepens until the embryo presents the appearance of a 



