THE INCUBATION OF HENS' EGGS 



183 



mouth and vent are formed. Tlie Ijones, niuseles, hlood, 

 reproductive and excretory or<i;ans develop from the 

 mesoderm, while the endoderm produces the linings of 

 the digestive tract and the resi)iratory and secretory 

 organs. 



Fio. 88 



Two embryos (tvvius) developing on one y(jlk. thirty-tuo iiuura old. 

 Maj^nilieil. (('ourti'sy of lo\\;i Ai^iicullinal I'l\[n'iiniciit, Stalion.) 



At al)out the twenty-fourtli hour of incubation a niemhrane 

 called the anuiion develops and gradually surrounds the 

 growing embryo. This amnion is filled with a colorless 

 fluid called the amniotic fluid, which serves as a i)rotection 

 from shock and allows the embryo to move about freely. 

 The growth of the amnion constricts the connection of the 

 yolk with the embryo to a narrow stem called the yolk 

 stalk. This stalk corresponds to the umljilical coril in 

 mammals. At about the thirtieth hour the heart of the chick 

 has developed far enough to be observed to beat. 



