DEVELOPMENT OF OTHER VERTEBRATES 



425 



eggs are laid in a mass of leaves and other dead vegetation, which 

 is heaped upon them by the parents and incubates the eggs by the 

 heat generated in its decomposition. 



Development of Birds. — As explained in the preceding para- 

 graph, a study of development in the birds is essentially an exten- 

 sion of one's knowledge of development in the reptiles, so close 

 are the resemblances in the embryology of these two classes of 

 vertebrates. Moreover, the presence of a large amount of yolk 



Fig. 222. — A lizard, the blue-tailed skink, Eumeces quinquilineatus, and its nest. 



A, adult male catching insect. B, young. C, female with eggs in a cavity of a rotten log. 

 (From Pearse, "General Zoology," copyright, 1917, by Henry Holt and Co., reprinted by 

 permission.) 



in birds and reptiles is important in the present summary of Ver- 

 tebrate Embryology, since it may be compared on the one hand 

 with the condition found in amphibians, and on the other with 

 that existing in mammals, in which the embryo is nourished by 

 diffusion from the blood of the parent. The development of the 

 domestic fowl has been chosen as representative of the birds, 

 because it has long been a favorite subject of investigation and is 

 perhaps better known than the development of any other animal. 

 The spermatozoa are formed in the testes (cf. Fig. 223) and pass 

 through the vasa deferentia to be stored in the seminal vesicles. 

 In the female, the organs on the right side, although present in 



