The Development of the Chick 113 



nected only with the alimentary canal (a 1 ) by a solid 

 pedicle. 



" Lastly, in N the yolk-sac (ys) is shewn being with- 

 drawn into the cavity of the body of the embryo. The 

 allantois is as before, for the sake of simplicity, omitted; 

 its pedicle would of course lie by the side of ys in the 

 somatic stalk marked by the usual dotted shading. 



" It may be repeated that the above are diagrams, the 

 various spaces being shewn distended, whereas in many 

 of them in the actual egg the walls have collapsed, and 

 are in near juxtaposition." 



We have now described, in some detail, the 

 condition of the egg at the time of laying, 

 and the changes that it has undergone previ- 

 ous to that time. 



To facilitate the study of the subject in the 

 laboratory, the development of the chick from 

 this point will be described by periods. For 

 example, the changes that take place during 

 the first day will first be described, then all 

 the changes of the second day, then of the 

 third day, and so on to the end of incubation, 

 at the twenty-first day. Since the changes 

 that take place after the first week are chiefly 

 those of growth, most of the space will be 

 given to the changes of the first four or five days. 



Before beginning the more detailed descrip- 

 tion of the changes that take place during 



