rves as a
sort of lung for 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 remain too long in contact with the
upper shell of the egg, it will become attached to it and will not
thereafter perform its functions.

The embryo thus enclosed in the egg finds its protection in the fact
that it is encased in a fluid contained 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 oxygen and throws off
its wastes through the instrumentality 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 organization is, however, not thoroughly
developed. If removed 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 enclosing the bubble of air at the blunt end of the
egg and get the first breath into its lungs. Now it gives a faint
peep, breaks the shell of the egg, and steps out into the open air.

I have given this somewhat lengthened description of the development
of the chick because of the light it throws upon the method pursued by
the mammals. The features which have been described in the case of the
chicken's egg could be as fully observed in the case of the turtle or
any of the other reptiles. Mammals are descended from the reptiles of
the Mesozoic, and whatever peculiarities there may be in their method
of producing their young must be derived from the reptiles. If we wish
to know how the earliest mammals produced their young, we can only
judge by the lowliest members of the group that live upon the earth
to-day. The most primitive of these is the so-called Duckmole, of
Australia. This little creature has habits not unlike those of the
muskrat. It burrows in the bank of a stream, and makes a nest at the
end of the burrow, where it lays its eggs. This is one of the very few
warm-blooded, hair-covered animals which still lays eggs. A little
higher in t