THE INCUBATION OF THE EGG 



167 



this difference for hens and pullets of the same strain and 

 under identical conditions was 1.77 pounds per hundred 

 eggs. 



Curtis* found that eggs increase in weight continuously 

 up to the end of the second laying season, but at a slower 

 and steadily decreasing rate. 



Development of the Chick. — The growth of the chick in 

 the egg occupies the incubation period, which is analogous 

 to the gestation period in mammals. It begins with the 

 fertilization of the ovum and continues until hatching or 

 birth, unless checked by some unfavorable condition. 



Fig. 79 



Appearance of a chick embryo after twelve hours in an incubator. 

 (Courtesy of Kansas Experiment Station.) 



Development begins by simple cell division in the living 

 portion of the egg, which is called the blastoderm, or, more 

 commonly, the germ spot. The first development is a clear 

 area called the area pellucida, and surrounded by a denser 

 ring of cells called the area opaca. The growth of the embryo 

 takes place in the area pellucida. In this area the so-called 

 primitive streak develops, which is the first sign of the chick's 

 body. This point in development is reached at about 



> Maine Bulletin No. '228. 



