236 



Basic Structure of Vertebrates 



MjP 



Fig. 199. Amphibian eggs. (A) Of frog, soon after laying. (B) Early larva of frog, 

 just before hatching. (C) Of the salamander, Cryptobranchus allegheniensis. (G) 

 Gelatinous layer; (L) larva; (OV) ovum. (A and C, approximately actual size; 

 B, enlarged.) (A and B, after Marshall; C, after Beese. Courtesy, Neal and Band: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



Eggs of mammals, with two exceptions, are minute, containing a 

 minimum of yolk. The exceptions are the duckbill (Ornifhorhynchus) 

 and the spiny anteater (Echidna) of the Australian region. These two 

 mammals, presumably of primitive type, lay large eggs encased in 

 tough shells. In general, these mammals are reptilian in their methods 

 of reproduction. Otherwise, mammalian eggs are of microscopic dimen- 

 sions (0.06 to 0.3 mm. in diameter). The egg (Fig. 201) is covered by a 

 delicate membrane (zona pellucida), external to which may be a 

 cellular membrane (corona radiata), both contributed by the ovary. 



Fertilization. Development is initiated by the "fertilization" of 

 the egg. A spermatozoon penetrates the egg (impregnation) and the 



neck of Utebra 



dense albumen 

 less dense albumen 

 vitelline membrane 



Fig. 200. Diagram representing a section of a hen's egg cut in a plane including 

 the long axis of the egg and passing through the blastoderm. (After Lillie. Courtesy, 

 Patten: "Early Embryology of the Chick," Philadelphia, The Blakiston Company.) 



