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- 



