CLASS MAMMALIA 419 



quarters in prolonged and profound sleep. Still others carry on various 

 activities during that time. It is while in their winter dens that female 

 bears bring forth their very small and partially developed young. 



447. Reproduction. — In all mammals fertilization is internal, the 

 sperm cells being introduced into the oviduct by copulation. I'he 

 egg cell of the mammal is small and possesses only a limited amount 

 of yolk. The development of the monotremes is essentially like that 

 of reptiles and birds. The mammals of the next group, the marsupials, 

 retain the embryo for a certain period of time within the uterus, though 

 it does not become attached by a placenta such as is found in all higher 

 mammals. 



Cleavage is apparently total and approximately equal. The egg cell 

 thus appears to be holoblastic although there are details in the develop- 

 ment which seem to indicate that it has been modified from a meroblastic 

 type. The egg cell divides first into two and then into four cells, equal 

 in size and normally arranged (Fig. 287). As cleavage continues, 

 however, the cells shift about and finally a structure is formed which 

 consists of an outer layer of cells, called the trophoderm, and an inner 

 cell mass. Gradually the trophoderm and the inner cell mass become 

 separated by a cavity filled with fluid and corresponding to a blastula 

 cavity, but the two remain in contact at one pole of the vesicle. The 

 trophoderm becomes attached to the wall of the uterine cavity and from 

 it finger-like projections or papillae grow into the uterine mucosa; these 

 serve to anchor it firmly to the wall of the uterus. 



From the inner cell mass is developed not only the entire embryo 

 but also the amnion, chorion, allantois, and yolk sac. In what may 

 be called a typical mammalian embryogeny the inner cell mass becomes 

 separated into two portions. In the upper portion develops an amniotic 

 cavity, while in the lower portion is formed an archenteron. The 

 amniotic cavity is lined with ectoderm, the archenteron with entoderm, 

 and from the cells between is formed the mesoderm. From the archen- 

 teron a yolk sac develops, which, however, does not contain yolk. Now 

 the development of the embryo proceeds in much the same manner as in 

 the development of a meroblastic egg cell. The entoderm together with 

 the splanchnic mesoderm forms the wall of the yolk sac, which is connected 

 with the enteron of the embryo by a slender yolk stalk. The ectoderm 

 and somatic mesoderm grow around the wall of the blastodermic vesicle 

 forming an amnion, which unites with the trophoderm to produce a 

 chorion. The chorion of mammals is, therefore, not homologous with 

 that of reptiles and birds (Sec. 409). Branching processes, larger and 

 more complex than those which attached the trophoderm, extend from 

 the chorion into the uterine tissues, which become thick and congested, 

 the two masses together forming the placenta. Between the two layers 

 of the mesoderm is the extra-embryonic coelom. 



