288 



GENP:RAL ZOOLOGY 



In the mammals structures for reproduction and the 

 nourishment of the young have attained far greater special- 

 ization than in any other group of vertebrates. The sexes 

 are always separate, the egg is fertilized within the female 

 and early stages of development take place within her body. 

 The most primitive mammals lay soft-shelled eggs some- 

 what like those of birds, but the majority have the egg 



body wall 



FIG. 106. Section through body wall and uterus of a placentate mammal to 

 show how the embryo is attached to the wall of the uterus. 



develop within the body and the young are nourished by 

 the mother for some time in many cases, both before and 

 after birth. The young of the marsupials are at first very 

 helpless. They are placed at birth in the marsupial 

 pouch on the ventral side of the abdomen, where they re- 

 ceive protection and are nourished from the mammary 

 glands. All mammals which do not lay eggs or place the 

 young in a marsupial pouch have a placenta (Fig. 106) of 

 some sort. This is a complicated structure which grows 



