Reproduction 



235 



Reptiles and birds produce eggs containing an enormous amount 

 of yolk (Figs. 200, 208). The protoplasm in these great eggs is aggre- 

 gated at one spot on the surface of the egg, marking the animal pole, 

 while the remainder of the egg is yolk nearly, if not quite, devoid of 

 protoplasm. The localized protoplasm (germ-disk: Fig. 208) appears 

 as a small white fleck on the surface of the yellow yolk. Before the 



&M- EGG-CASE 



Fig. 198. Egg-case 

 of small shark. (T) 

 Tendrils coiled around 

 branches of a horny 

 (gorgonian) coral. 

 (About half actual 

 size.) (Courtesy, Neal 

 and Hand: "Chordate 

 Anatomy," Philadel- 

 phia. The Blakiston 

 Company.) 



ovum is fertilized, the germ-disk contains a single nucleus, consistent 

 with the fact that the ovum is one cell, however much gorged with 

 yolk. But as a dynamic thing — i.e., physiologically — the germ-disk is 

 the essential cell. These large eggs are invested by a tough vitelline 

 membrane external to which may be more or less nutritive albumen 

 (the "white" of a hen's egg) and an outer shell which in most reptiles 

 is of a leathery texture, but in crocodiles, alligators, and birds is highly 

 calcified and, therefore, hard and brittle. Against the inner surface of 

 the shell lies a thin, fibrous shell-membrane. 



