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 amamma. 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 
