206 THE MEANING OF EVOLUTION 
colored disk which floats on the top of the yolk, form 
the nourishment. The disk alone is the living organ- 
ism. In the earliest stages the embryo 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 flattened bag which floats above the embryo 
against the upper side of the shell. This bag is 
called the allantois, and serves as a sort of lung for 
