REPRODUCTION. 307 



life, since through its placental vessels it exchanges its effete 

 products for the oxygen and nourishment contained in the 

 mother 's blood. ' * 



Birth. While the ovum is being developed into a human 

 being by division of the original cell of the fertilized ovum, the 

 uterus becomes very much enlarged, and its walls increase in size 

 by the growth of muscular tissue. At the end of approximately 

 280 days from the date of impregnation of the ovum, the devel- 

 opment is complete and birth takes place. This consists in the 

 expulsion of the fo?tus by muscular contractions of the uterus. 



Directly the child is born, the placenta begins to separate from 

 the uterine wall and is soon expelled. The child deprived of. its 

 placental nourishment must now begin an independent life. It 

 must take in its own oxygen and give off carbon dioxide by its 

 respiratory organs. It must take its food through the alimentary 

 canal, and excrete its waste products through its kidneys. 



