288 



REPRODUCTION OF ANIMALS 



cause the formation of embryos. Sperm cells must be kept moist 

 in order to function. In the case of frogs and fish, fertilization is 



external and the water keeps the cells 

 moist. In the case of animals that do 

 not live in the water, such as insects, 

 birds, or mammals, the egg cell is re- 

 tained within the body of the female, 

 and the union of the egg and sperm is 

 internal. Thus the danger of the gam- 

 etes drying is averted. The embryo 

 develops outside of the body when the 

 fertilization is external. The water 

 also keeps the developing embryo 

 moist. When the fertilization is inter- 

 nal, the embryo may partially develop 

 or may completely develop within the 

 body of the female. For example, the 

 eggs of the insects start developing 

 within the female, but later they are de- 

 posited in the ground or on plants arid 

 the development continues there. 



Amer. Museum of Natural History 

 In the birds, the original egg cell 

 and yolk become surrounded by 

 other materials during the passage 

 down the oviduct. It becomes sur- 

 rounded by a coating of albumen, 

 the white of the egg, which is se- 

 creted by the glands of the oviduct. 

 A lime coating which forms the shell 

 is then secreted around the whole 



^^^ " ^membrane, 



In the case of birds, 



.A section through a chicken's egg shows the tiny em- 



the embryo Starts an in- bryo attached to the yolk. Surrounding the yolk is the 



I , , albumen which protects the embryo from shock. Two 



temal development, thin membranes within the shell form an air chamber at 



The fertilized egg re- one end ' 



ceives a huge deposit of yolk, then albumen is spread around it, 



and finally it is enveloped in a hard shell of lime. The yolk 



