HOW THE MAMMALS DEVELOPED 207 
the developing chick. The shell is porous enough to 
allow air to pass through it. The blood vessels of 
the allantois take in oxygen and give out carbon 
dioxide through the porous shell. The blood thus 
altered is returned to the chick and serves its life 
purposes. One of the reasons why the chicken must 
turn its eggs in the nest is that, if the allantois re- 
main too long in contact with the upper shell of the 
egg, it will become attached to it and will not there- 
after perform its functions. 
The embryo thus enclosed in the egg finds its pro- 
tection in the fact that it is encased in a fluid con- 
tained in the amnion. It draws its nourishment from 
the yolk upon which it lives and the nourishment is 
transmitted to it by blood vessels. It draws its oxy- 
gen and throws off its wastes through the instru- 
mentality of the allantois, which covers it over. Day 
by day the chick becomes larger, day by day it grows 
to look more like what it is to be. By the nineteenth 
day it appears to be complete. Its nervous organiza- 
tion is, however, not thoroughly developed. If re- 
moved from the shell the chick still is indisposed to 
stand upon its feet or to run about. If allowed to 
remain in the egg until the twenty-first day, the chick 
will be able to push its beak through the skin enclos- 
ing the bubble of air at the blunt end of the egg and 
get the first breath into its lungs. Now it gives a 
