BEPKODUCTIVE FUNCTIONS. 907 



of birds, in contradistinction to what holds in mammals, segmentation 

 is partial, while the remainder of the yelk contained within the umbilical 

 vesicle, and consequently communicating with the intestine of the bird, 

 serves for its nutrition ; as a consequence the umbilical vesicle persists 

 during the entire period of incubation and even up to the time when the 

 bird issues from the shell. 



The heat necessary for the development of the embiyo within the 

 egg is, as a rule, supplied by the body of the mother, but, as is well 

 known, eggs may be artificially incubated, it only being necessary 

 to place them at a constant temperature of from 35 to 40 C., turning 

 them each day. 



In mammals the female nourishes its young for a variable period 

 after birth through the secretion formed by the mammary glands. 



In mammals the different acts of generation closely correspond with 

 similar processes in man, the principal differences lying in the number 

 of the young, the duration of gestation, the frequency of the acts of 

 reproduction, and certain anatomical peculiarities relative to the mode 

 of adherence of the foetus or foetuses to the uterine cavity. Among 

 mammals some bear but one young at a time. These are the cow, mare, 

 ass, stag, elephant, and monkey ; the bear, the roebuck, the castor, the 

 marmot, and the guinea-pig three to four; the lion, the tiger, and the 

 leopard four to five ; the dog, the fox, the wolf, and the cat five to six ; 

 the rabbit and the water-rat six to eight ; the pig and the rat as many 

 as fifteen. 



The duration of gestation is three weeks in the guinea-pig, four 

 weeks in the rabbit and hare, five weeks in the rat and marmot, six 

 weeks in the ferret, eight weeks in the cat, nine weeks in the dog and fox, 

 ten weeks in the sloth, fourteen weeks in the lion, seventeen weeks in the 

 castor and sow, twenty-one weeks in the sheep, twenty-two weeks in the 

 goat, twenty-four weeks in the roebuck, thirty weeks in the bear, thirty- 

 six weeks in the stag, forty-one weeks in the cow, forty-three weeks in 

 the mare, the ass, and the zebra, forty-five weeks in the camel, and one 

 hundred weeks in the elephant. 



The number of young borne by mammals is capable of being modi- 

 fied under different conditions. Animals which in a state of nature, 

 copulate but once in a year, when reduced to a state of domestication 

 enter anew into heat and may copulate a short time after the previous 

 birth ; this is, without doubt, due to the more abundant nourishment sup- 

 plied to such animals in a state of domestication. The mare may pass 

 into heat ten or twelve days after the birth, the cow after twenty days. 



The number of young borne by mammals is principally subordinate 

 to the duration of gestation. Small mammals, which carry their young 

 but a short time, as a rule bear more frequently than those in which 



