752 



AVES. 



the allantoid apparatus is not developed. The egg in these lower tribes 

 contains only the yelk and the cicatricula ; it swells from absorbing the 

 surrounding water, and the fetus is developed upon the surface of the 

 yelk, the latter, which, as in Birds, communicates with the intestine, 

 being slowly received into the abdominal cavity. 



Fig. 379. 



Viscera of mature chick. 



(2142.) The subsequent changes that occur in the circulatory system 

 of a bird, namely, the obliteration of the foramen ovale and of the ductus 

 arteriosi, whereby the pulmonary and systemic circulations become quite 

 distinct, are similar to those which take place in the Mammiferous foetus, 

 and will be described in the next chapter. 



