70 STRUCTURE OF THE VERTEBRATES 



differentiation of the primitive germinal layers into tissues and 

 organs, and their individual histories are traced in Part II. 



Continued growth increases the size of the embryo and de- 

 creases the amount of yolk. At the time of the completion of the 

 amnion the embryo is a small structure on top of a large yolk 

 mass; but by the time of hatching their relative sizes have been 

 reversed and the yolk sac is a small mass on the ventral side of 

 the young animal. In hatching the shell and surrounding mem- 

 branes are broken and the latter are sloughed off in a short time. 



Laboratory Study. The great majority of laboratories find it 

 impractical to get reptiles for a study of embryology and use 

 the chick instead. The bird is essentially similar to the reptile in 

 its developmental processes, and due to the ease with which fer- 

 tilized hen's eggs are obtained and hatched in the laboratory, 

 they make ideal material for a study of meroblastic cleavage 

 and the principles of amniote development. For this reason the 

 illustrations used show chick development. 



