542 DEVELOPMENT OF FIFTH DAY 



On the thirteenth day the nails and scales appear on the toes, and 

 by the sixteenth day nails, scales and beak are firm and well developed. 

 On this day also the cartilaginous skeleton completes its growth, and 

 various centers of ossification make their appearance. 



By the sixteenth day the white of the egg has disappeared and the 

 mesoderm has divided completely into the splanchnopleure and somato- 

 pleure all the way around the yolk. 



On the nineteenth day the remains of the yolk are drawn into the 

 body cavity of the embryo. 



The embryo begins its development originally by lying with its 

 axis transverse to the long axis of the egg, but by the fourteenth day it 

 turns so that its head is toward the air space at the larger end of the 

 egg, and at about the twentieth day the chick's beak is pushed through 

 the inner covering of the air space so that it can now begin using its 

 lungs. It is at this time that the pulmonary circulation begins. The 

 blood stops flowing into the umbilical vessels, and the allantois conse- 

 quently shrivels up and is left inside the shell as the chick pecks its way 

 out. 



