HOW THE MAMMALS DEVELOPED 211 



formed allantois mesh in with those of the thickened 

 parent wall. Thus the blood vessels of the mother 

 are brought into close contact with those of her off- 

 spring. Her blood seeps over into the transformed 

 allantois which is now called a placenta. From this 

 it is handed over to the offspring, which thus receives 

 from the mother her blood, and returns its own used 

 blood for enrichment and purification. So the allan- 

 tois of the reptile has become the placenta of the 

 mammal. In the first instance it served only as an 

 organ of respiration. Now it has come to supply the 

 embryo with rich blood containing both food and 

 oxygen derived from the mother. After the offspring 

 is born this thickened pad breaks loose, and subse- 

 quently is also extruded from the body, forming what 

 is known as the afterbirth. 



Thus far we have spoken of the change in the 

 method by which the young are brought to such a 

 stage of development that they can stand the outer 

 air. One of the improved differences between the 

 mammals and other animals lies in the method by 

 which they nourish their young for some time after 

 birth. The very word mammals signifies an animal 

 who is in the true sense of the word a mamma. This 

 name for mother is given to her because of the fact 

 that she possesses what are technically known as mam- 

 mary glands, or, in simpler language, breasts. It 



