112 PRIMITIVE ANIMALS 



canal of the embryo, so that the yolk can pass into 

 the gut of the embryo and so be used as food. 

 Numerous blood-vessels arising from the umbilical 

 veins and arteries pass out of the embryo and spread 

 over the yolk-sac, and by their assistance the food 

 material of the yolk is taken up into the general 

 circulation of the embryo and distributed all over 

 its body. As the embryo increases in size and its 

 organs become larger and more completely developed, 

 the yolk-sac becomes included in its alimentary canal, 

 and a few days after hatching it is completely 

 digested, and the young reptile or chick must be 

 supplied with food from without. 



In this process development takes place within 

 the egg-shell entirely at the expense of the yolk, 

 which has been stored by the parent in the egg 

 before fertilization. 



In the case of a typical Mammal, such as a rabbit, 

 a dog, or a human being, an entirely different mode 

 of foetal nutrition takes place. The egg when it 

 leaves the ovary is exceedingly small and contains 

 a minimal quantity of yolk. On attaining to the 

 oviduct and being fertilized, it passes into the uterus, 

 where it becomes attached and finally embedded in 

 the uterine wall. Here it begins to develop, and 

 it is at first nourished by the secretion of glands 

 in the uterine wall of the parent. 



It is necessary, in order to follow how the 



