THE INCUBATION OF THE EGO 



169 



is true also. If the temperature is somewhat above 103° F., 

 germination proceeds more rapidly. For instance, twenty- 

 four hours at 104° F. to 107° F. gives a chick which is equal 

 in development to one incubated for three days at 103° F. 



As soon after laying as the egg is placed at a normal incu- 

 bating temperature, cell division proceeds rapidly, and the 

 cells form themselves into three layers which, from without 

 to within, are referred to as the ectoderm, mesoderm, and 

 endoderm. These three layers are the beginnings of three 

 distinct divisions of the body. 



Fig. 81 ' 



Appearance of a chick embryo after twenty-four hours in an incubator. 

 (Courtesy of Kansas Experiment Station.) 



From the ectoderm, the skin, feathers, beak, claws, ner- 

 vous system, lens, and retina of the eye and linings of the 

 mouth and stomach are formed. The bones, muscles, blood, 

 reproductive and excretory organs develop from the 

 mesoderm, while the endoderm produces the linings of 

 the digestive tract and the respiratory and secretory 

 organs. 



At about the twenty-fourth hour of incubation a membrane 

 called the amnion develops and gradually surrounds the 

 growing embryo. This amnion is filled with a colorless 



