170 



POULTRY PRODUCTION 



fluid called the amniotic fluid, which serves as a protection 

 from shock and aUows 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 umbilical cord in 

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

 has developed far enough to be observed to beat. 



Fig. 82 



Two embryos (twins) developing on one yolk, thirty-two hours old. 

 Magnified. (Courtesy of Iowa Experiment Station.) 



Near the middle of the second day a blind sac begins to 

 push out from the posterior end of the alimentary tract, 

 which grows rapidly until it completely covers the embryo 

 at about the ninth day and has come in close contact with 

 the shell membrane. It is abundantly supplied with blood- 

 vessels and, although it is outside the body of the embryo, 

 serves as the organ of both respiration and excretion until 



