The Development of the Chick 115 



Let these superimposed circles represent a 

 greatly enlarged blastoderm that has been re- 

 moved from the yolk to which it was originally 

 attached. The upper layer of cloth will repre- 

 sent the ectoblast, the bottom layer will rep- 

 resent the entoblast, and the two similarly 

 colored layers in the middle will represent the 

 two layers of the mesoblast after their separa- 

 tion. The method of the formation of these 

 three germ layers, during the first day of incu- 

 bation, will be described under the head of the 

 first day's development. 



As the yolk takes no actual part in the forma- 

 tion of the embryo, other than as a supply of 

 food for the growth of the constantly enlarging 

 chick, it may be omitted in our model. 



Now spread the cloth blastoderm upon a 

 table and place under its centre a small object, 

 such as a bottle. If now the fingers of one 

 hand be pushed under one end of the bottle, 

 carrying, of course, the three germ layers with 

 them, we shall have represented the formation 

 of the head fold that is represented and de- 

 scribed in Figs. 37 and 38. By pushing under 

 the cloth at the other end of the bottle, in the 

 same way, we may represent the formation of 

 the tail fold ; and in a like manner the lateral 



