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 devielop 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 and 

 the development continues there. 



. Amer. Museum 0/ Natural History 

 In the birds, the original egg cell ^^____. ^^^ SViell 



and yolk become surrounded by "^ ^^^*- **5X1 



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 



mass. 



membrane. 



In the case of birds, 

 the embrvo starts an in- 

 ternal development. 

 The fertilized e^g re- 

 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 



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

 bryo attached to the yolk. Surrounding the yolk is the 

 albumen which protects the embryo from shock. Two 

 thin membranes within the shell form an air chamber at 

 one end. 



