DEVELOPMENT OF BIKDS AND MAMMALS 24? 



Zona Ectoderm Int. cell mass 



During the incubation of the chick, these vessels are highly 

 developed. The blood to the allantois is carried by special 

 allantoic or umbilical arteries, and the allantoic veins join the 

 vitelline veins. 



The limbs are developed as buds of the outer wall and 

 receive derivatives of the myotomes which furnish them with 

 muscles. The forward pair are the wings, the hinder pair 

 the legs. 



Towards the end of the period of incubation the yolk sac 

 passes into the body of the chick, and its endoderm is 

 absorbed in that of the intestine. 

 Shortly afterwards the umbi- 

 licus closes and the chick 

 pierces the air space at the 

 broad end of the egg and 

 begins to use its lungs. It is 

 hatched by the beak breaking 

 the shell, and leaves behind it 

 the egg together with the allan- 

 tois, amnion, and the serosa. 



Development o Mammalia. 

 The eggs of mammals are very 

 small and undergo holoblastic 

 segmentation. This and other 

 modifications are associated 

 with the egg being retained in 

 the oviduct and receiving its 

 food and oxygen from the blood 



system of the mother. It has become plain, however, from 

 the work of Caldwell, Semon, and of Wilson and Hill, that 

 the lowly oviparous monotreme mammals have meroblastic 

 eggs, and, considered with the marsupials, are in their early 

 stages intermediate between reptiles and placental mammals. 



The eggs of the placental mammals measure only 0*1 to 

 2 mm., and, as has been said, are holoblastic in segmentation. 

 The first polar body is liberated before the egg leaves the 

 ovary, and the second after the spermatozoon has entered 

 the egg. It is covered in an egg membrane, the zona radiata, 

 and in the upper part of the oviduct it receives a slight 



End. Ect. 



Ect. 



Uterine 

 epith. 



FIG. 126. Segmentation of the 

 mammalian egg. After van 

 Beneden. 



