BRANCH CHORD AT A: CLASS MAMMALIA 3 8 7 



y. v 



Development and life-history. All mammals except 

 the Monotremes give birth to free young. The two 

 genera of Monotremes produce their young from eggs 

 hatched outside the body; Tacliyglossus lays one egg 

 which it carries in an external pouch, while Ornithorhyn- 

 ''hus deposits two eggs in its burrow. The embryo of 

 other mammals develops in the lower portion of the egg- 

 lube, to the walls of which it is intimately connected by 

 a membrane called the placenta. (In the kangaroos and 

 opossums, Marsupialia, there is no placenta.) Through 

 this placenta blood-vessels extend from the body of the 

 mother to the embryo, the young developing mammal 

 thus deriving its nourishment directly from the parent. 



The duration of gestation (embryonic or prenatal 

 development in the mother's body) varies from three 

 weeks with the mouse, eight weeks with the cat, nine 

 months w r ith the stag, to t\venty months with the elephant. 

 Like the birds, the young of some mammals, the carni- 

 vores for example, are helpless at birth, while those of 

 others, as the hoofed mammals, are very soon able to run 

 about. But all are nourished for a longer or shorter time 

 by the milk secreted by the mammary gland of the 

 mother. 



Habits, instinct, and reason, Despite the wonderful 

 examples of instinct and intelligence shown by many 

 insects and by the other vertebrates, especially the birds, 

 it is among mammals that we find the highest develop- 

 ment of these qualities and of reason. In the wary and 

 patient hunting for prey by the carnivora, in the gregarious 

 and altruistic habits of the herding hoofed mammals, in 

 the highly developed and affectionate care of the young 

 shown by most mammals, and in the loyal friendship and 

 self-sacrifice of dogs and horses in their relations to man, 

 we see the culmination among animals of the development 

 of the functions of the nervous system. In the character- 



