162 GENERAL ZOOLOGY 



with the egg, and later the tissues of the embryo, to supply the 

 latter with nutrition and oxygen and to remove carbon dioxide 

 and other waste products of metabolism. The eggs of such 

 mammals are small in size and contain an insufficient amount of 

 stored nutrition in the form of yolk to carry development beyond 

 the very earliest stages. 



Both the tissues of the uterus and the tissues of the embryo 

 participate in the formation of a placenta, which is an organ that 



40 "i in.. 



Fig. 107. — Embryo sharks of a viviparous species, Mustelus mustelus, attached 

 to the walls of the uterus. The expanded end of each umbilical cord forms a 

 placenta-like structure, resembling that of mammals, through which the young 

 are nourished. (From Shull, LaRue, and Ruthven, Animal Biology, after Fowler.) 



develops during the period of gestation. The placenta is thus a 

 composite organ which differs in detail in different mammals. 

 The maternal and embryonic blood streams are never confluent 

 in the placenta, but they are brought close enough together to 

 permit the diffusion of gases and substances in solution through 

 the intervening tissue. The mammalian egg is released from 

 the ovary by the rupture of the ovarian wall and enters the 

 oviduct, in the upper part of which fertilization takes place. The 

 fertilized egg begins to develop as it passes down the oviduct to 

 the uterus where it becomes attached more or less firmly to the 

 lining of the uterus. The placenta then begins to develop and 

 with it the development of an umbilical cord through which blood 



