DEVELOPMENT OF THE EMBRYO 



267 



the ninth day, later the muscles of the wings are visible, and by 

 the eleventh day the arteries are quite distinct. Up to the end 

 of the first week the embryo lies very still, after which it shows 

 unmistakable signs of voluntary motion. The yolk is now per- 

 ceptibly thinner, for the growing embryo draws heavily upon it 



(Courtesy Million Egg Farm) 



Fig. 169. — Testing hatching eggs. 



for nourishment. The feathers are well developed by the four- 

 teenth day; on the eighteenth day the first cry is usually heard. 

 On the day following the yolk should be nearly all taken into the 

 body. The beak of the chick then breaks through the membrane 

 into the air cell, after which it soon pips the shell and extricates 

 itself. 



