he scale stand the kangaroo and the opossum. These
creatures keep the egg inside of the body until it is hatched. But
this happens in so short a time that the young animal is exceedingly
immature and as yet unable to stand the outside air. Accordingly there
is a double fold of skin on the abdomen of the mother, covering her
breasts. This forms a suitable resting place into which these young
are conveyed as soon as they are born and from which they do not
emerge for many days. The little creature instantly fastens upon the
nipple of the mother, keeping its mouth constantly in this position.
At intervals the muscles of the breast force the milk into the mouth
of the young, which is still too undeveloped to suck for itself. As it
gets older the little opossum or kangaroo emerges from the pouch in
the pleasanter part of the day and in the absence of danger. It
returns to the mother's pocket as soon as it becomes cold or a cry
from its parent warns it of its defenseless position.

These creatures are the lowliest of the class upon the earth. The
great majority of all mammals have elaborated a far finer plan, in
which the young are retained within the body of the parent until they
are quite able to stand the air. The length of this time varies in
different mammals from a few weeks to more than a year. The egg must
be fertilized before it leaves the body of the parent. If it should
fail in this it simply passes out and is wasted. If the fertilizing
cell reaches the egg before it has progressed far down the tube it
begins its development. The embryo forms for itself the sort of head
and tail and gill slits which would have served its fish or its
tadpole ancestor. Its limbs develop as little buds indistinguishable
from similar buds that would have formed fins for the fish or wings
for the bird.

Around the embryo there forms a sac, the amnion filled with a fluid
which serves to protect the young mammals exactly as the growing chick
was protected. Under the forming creature there hangs a small but
empty yolksac. This is an actual remnant, a reminder of the past, when
the eggs of the mammals were also packed with yolk and the growing
embryo secured its nourishment exactly as does the maturing chick. But
a new method has been provided for the mammal, and consequently the
yolksac, though it has not entirely disappeared, has no nutritive
content for the growth of the embryo.

The allantois of the chick now gains a new development