HOW THE MAMMALS DEVELOPED 205 
Almost everyone has tried to dissolve some substance 
in water in a vial. If the bottle be filled with fluid 
to the top and corked it is very difficult to shake up 
the contents. Even vigorous agitation produces little 
movement of the material on the inside. If we wish 
to shake up the solid with water the bottle must be 
left partly empty. The brain of a human being is 
protected by just the same device. If it simply lay 
within the skull the first fall would mash the gray 
substance against the side of the cavity. To prevent 
this calamity the bony case is made somewhat larger 
in capacity than the brain itself, and the space be- 
tween the two is filled with a watery fluid. This 
serves to prevent jars and shocks. In the hen’s egg 
the same plan is pursued. The embryo lies on the 
inside of a bag considerably larger than itself. This 
sac, called the amnion, is filled with a watery fluid. 
With such a protection only the most severe shock 
to the egg would sufficiently jar the embryo to do it 
any harm. The ordinary experiences of an egg leave 
it undisturbed. 
Every living creature requires a constant supply of 
food and of oxygen. The embryo is a living crea- 
ture, and is no exception to the rule. It needs an 
abundant supply of easily assimilated food and of 
oxygen. When the hen’s egg is first laid the en- 
tire contents, with the exception of the little light- 
