MAMMALS AND MAN . 99 



mother. Thus it grows, literally a parasite on the mother 

 just as the sporophyte of the moss is a parasite on the 

 gametophyte. 



As the heart and blood vessels of the young mammal 

 develop, the latter run out to these connections with the 

 mother just as those of the chick did over the foodstuff in 

 the Ggg. Oxygen as well as food is thus received from the 

 blood of the mother, and the carbon dioxide and other 

 wastes are returned to it. 



This internal development continues for very different 

 lengths of time in different animals. In one of the groups 

 of mammals, the marsupials, which includes the opossum 

 and kangaroo, the young are carried only for a short time. 

 They are born in a very immature condition. They are 

 then placed in an outside pouch which contains the milk 

 glands. Here the young remain until they are mature 

 enough to take some care of themselves. In the higher 

 mammals, the period within the uterus varies from a few 

 days to a year or more. For the large mammals it is 

 usually a little less than a year. In these the development 

 at birth is greater than it is in the marsupials. 



5. Birth and its Changes. Birth in mammals may be 

 said to correspond to hatching in the birds. The young 

 mammal passes through all its embryonic stages in the 

 uterus and has all its principal organs ready for use when 

 born. While it is living one kind of life (that within the 

 mother) it is being made ready for a very different one. 

 The change in birth is like that in hatching, a very sudden 

 one. The change of life in the tadpole on the other hand 

 comes very gradually. 



Before birth the young are surrounded by the body of 

 the mother and have a constant temperature- At birth 

 they are brought into a very much colder and more 

 changeable temperature. Before birth all the food and 

 oxygen are taken up by the blood from the mother's blood. 

 At birth this supply ceases, and the lungs for the first 



