206 THE MEANING OF EVOLUTION 



colored disk which iloats on the top of the yolk, form 

 the nourishment. The disk alone is the living organ- 

 ism. In the earliest stages the emljryo receives its 

 food by simple absorption from the yolk. As the 

 chick increases in complexity the yolk at first grows 

 swampy, with fluid trickling here and there through 

 the more solid portions. Thin walls form about these 

 little streams, thus producing blood vessels which 

 cover the entire surface of the yolk. These absorb the 

 nourishment and turn it over to the embryo. As the 

 latter grows in size both the yolk and white dimin- 

 ish. The embryo soon becomes larger than the remain- 

 ing yolk and is attached to it by a cord filled with 

 blood vessels which enter the chick near the center 

 of its body. The abdominal wall has an opening at 

 this point. One of the later occurrences in the life 

 of the chick, before it breaks through the egg, is to 

 have the last remnant of the yolk and its sac slip to 

 the inside of the abdomen, which then completely 

 closes over it. 



As yet, we have seen no arrangement for furnish- 

 ing air to the chick. At the same point at which the 

 blood vessels from the yolk enter the chick, another 

 set of vessels pass in and out. These are attached to 

 a large fiattened bag which fioats above the embryo 

 against the upper side of the shell. This bag is 

 called the allantois, and serves as a sort of lung for 



