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 between 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 creature, 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 entire contents, with the
exception of the little light-colored disk which floats on the top of
the yolk, form the nourishment. The disk alone is the living organism.
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 diminish.
The embryo soon becomes larger than the remaining 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 furnishing 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 se