and an altered
function. In the case of the chick it floats against the shell of the
egg and absorbs oxygen through the shell. Inside the body of the
mammal this is impossible, because the air is too far away. No shell
is formed about the egg because it is not to be laid. The tube of the
parent's body in which the egg lies becomes thickened at the point of
contact with the egg. It grows spongy and full of blood vessels.
Meanwhile the allantois is also growing spongy. These two tissues are
so closely pressed against each other that the blood vessels of the
transformed allantois mesh in with those of the thickened parent
wall. Thus the blood vessels of the mother are brought into close
contact with those of her offspring. Her blood seeps over into the
transformed allantois which is now called a placenta. From this it is
handed over to the offspring, which thus receives from the mother her
blood, and returns its own used blood for enrichment and purification.
So the allantois of the reptile has become the placenta of the mammal.
In the first instance it served only as an organ of respiration. Now
it has come to supply the embryo with rich blood containing both food
and oxygen derived from the mother. After the offspring is born this
thickened pad breaks loose, and subsequently is also extruded from the
body, forming what is known as the afterbirth.

Thus far we have spoken of the change in the method by which the young
are brought to such a stage of development that they can stand the
outer air. One of the improved differences between the mammals and
other animals lies in the method by which they nourish their young for
some time after birth. The very word mammals signifies an animal who
is in the true sense of the word a mamma. This name for mother is
given to her because of the fact that she possesses what are
technically known as mammary glands, or, in simpler language, breasts.
It would seem as if here we had an entirely new organ. No other
animal gives nourishment to its young in such fashion; all mammals do.
What is the origin of the habit? How did the organ arise?

A part of an animal's body that has the power to gather material from
the blood and pour it out in the shape of fluid is known as a gland.
Sometimes a whole organ does nothing else. Sometimes small glands are
scattered through, or over, the surface of another organ. There are
two kinds of glands in the skin of the mammal. The best known and most
frequen