REPRODUCTION OF HIGHER ANIMALS 



289 



A photomicrograph of a fertile egg that has been incubated for 

 two days. Cell differentiation and specialization have taken 

 place to the extent that the brain, heart, and blood vessels can 

 be recognized. 



and albumen serve as food for the developing embryo and at 

 the same time keep it moist. The shell permits air to enter, 

 but prevents the 



evaporation of x ' iiPTf*ft|%:*r*1 



moisture. Then 

 the mother bird 

 lays the egg, and 

 the development 

 of the embryo con- 

 tinues in the egg, 

 but external to the 

 mother's body. 



In mammals, for 

 example the rabbit, 

 the entire develop- 

 ment is internal. 

 When the embryo 

 is completely developed into a young animal, very much like the 

 adult, it is born. The word " birth " is used to describe the process 



of the little individual coming 

 forth after complete internal de- 

 velopment. This is in contrast 

 with hatching, which is the com- 

 ing forth of an animal from an 

 egg. During its internal devel- 

 opment, the mammalian embryo 

 is kept moist and warm within 

 the body of the mother. It is 

 fed by absorbing food from blood 

 vessels which pass through the 

 placenta of the mother organism. 

 There is a similarity in function between the placenta of flow- 

 ering plants and the placenta of mammals. They both are the 



i. -^ ... plorcentcc 



The unborn mammal is suspended in the 

 uterus by the umbilical cord. Surrounding 

 fluids serve as protection against shock. Food 

 for development is absorbed by osmosis 

 through the placenta. 



