98 REPRODUCTION 



3. The Gametes in Mammals. As in birds, there is no 

 reproduction in mammals except by the formation of eggs 

 and sperms. The sexes are always separate. The female 

 reproduces by small eggs, much smaller even than those 

 of the frog. Sperm cells are produced by the males in 

 testes just as in frogs and birds. 



The mammals are like the birds in that the eggs are 

 fertilized within the body of the mother. This is not 

 because of a shell about the egg as in the case of birds. 

 It is rather because the embryo when fertilized is going to 

 remain in the body of the female and there develop. It 

 is incubated within the mother instead of in a nest. For 

 this reason the sperms must be inserted into the special 

 female organ, the vagina, by a special act of mating 

 called copulation. From the vagina the sperms ascend 

 and meet the eggs as they come down from the ovaries. 

 The meeting place of sperm and egg is usually the organ 

 in which the young are retained during early development. 

 This organ is the uterus, or womb. It is a special organ 

 formed by a great enlargement of the oviducts or tubes 

 that lead from the ovaries to the outside. 



Fertilization itself differs in no way from that studied in 

 the lower plants and animals. 



4. Early Dievelopment of the Embryo. After fertilization 

 the fertilized egg (embryo) divides into two cells, then 

 into four, eight, sixteen, etc., just as in other animals. By 

 steps similar to those studied, the embryos become 

 differentiated into nerve, and muscle, and cartilage and the 

 other tissues. These steps need not be followed again. 

 The point which makes the mammal embryo so peculiar 

 and interesting is this: there is little food in the egg, 

 hence it cannot live as long as the frog's egg or the hen's, 

 on the strength of its own food. So, very soon after 

 fertilization the embryo attaches itself by root-like out- 

 growths to the tender walls of the uterus. Through these 

 connections it takes up nourishment from the blood of the 



