CHAPTEK XI 



THE DEVELOPMENT OF BIRDS AND MAMMALS 



Phylum VERTEBRATA 



Sub-Phylum CRANIATA 

 Class AVES 



MAMMALIA 



EEPTILES, Birds, and Mammals are grouped under the name 

 Amniota for the reason that the embryo is enclosed in an 

 envelope, called the amnion. The early history of the chick 

 illustrates the formation of the amnion and of other embryonic 

 membranes. A study of the development of birds, still better 

 that of reptiles, is a necessary preliminary to an under- 

 standing of the changes induced by the uterine period of 

 life of mammals. The material for a study of the embryology 

 of the chick (Gallus) is easily obtained ; but although birds 

 are very consistent in their development, important points 

 are better displayed in the embryology of the less modified 

 sea birds. 



The eggs of birds and reptiles, like those of the Elasmo- 

 branchs, are large and meroblastic, and in the case of some 

 of the extinct species of birds they reached a gigantic size. 

 The nucleus is superficial, and the cytoplasm is distended by 

 a large provision of food material, the yolk. It is covered by 

 a vitelline membrane, and during its passage down the oviduct 

 it is coated with layers of albumen, the white of the egg. The 

 egg during its descent is rotated around the oviduct, and the 

 albumen is coiled in front and behind the yolk into spiral 

 processes termed chalazae. In the lower part of the oviduct 

 the outer layers are hardened to form a double shell membrane, 



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