THE RISE OF THE MAMMALIA 119 



vascularization of the placenta, which is formed all 

 round the trophoblast, is not brought about by the 

 allantois but by a mesodermal stalk developed much 

 earlier. We cannot, however, enter here into the 

 various details of this subject, many of which are 

 difficult to observe and still more obscure to in- 

 terpret. 



The period of gestation varies considerably in 

 the different orders of Mammals according to the 

 size and state of development at birth of the young 

 and to the number of young in a litter. 



After birth the young Mammal is nourished in a 

 characteristic way by its mother, namely, by suckling. 

 The Mammals are characterized by the possession of 

 peculiar skin-glands, some of which secrete sweat 

 for the regulation of the body temperature ; others 

 secrete sebum, an oily material for keeping the hair 

 moist and pliable. Still others, probably homologous 

 and of an essentially similar nature to the foregoing, 

 are collected into special regions of the thorax or 

 inguina and secrete the nutrient fluid, milk. These 

 mammary glands, as they are called, open on to 

 special papillae, the teats, from which the young 

 after birth can suck a copious supply of milk. The 

 number of teats possessed by the female of any 

 Mammal is proportional to the greatest number of 

 young generally produced in a litter. Milk con- 

 stitutes a complete diet for the suckling young, 



