BIRDS 



ones being yellow, which vary in size from tiny dots to full- 

 sized yolks (Fig. 54). If any one of these is examined care- 

 fully with a microscope, a single egg-cell may be found. After 

 the yolk has attained its full size and the egg-cell has been 



FIG. 54. Ovary of hen, and egg in egg-tube. 



fertilized, it receives its coating of white, and the whole is 

 covered with the membranes and the shell. 



Immediately after fertilization takes place, by the process 

 of cell division many cells 

 are formed . At the time 

 the egg is laid, the chick 

 embryo appears as a tiny 

 white spot on the surface 

 of the yolk when the egg 

 is opened (53, 4). Fur- 

 ther development of this 

 embryo, however, can- 

 not take place unless the 

 egg is kept warm. This 



is brought about when 



FIG. 55. Egg of hen, showing embryo 

 chick on surface of yolk. (Beebe. "The 

 Bird.") 



