pment, denoting
it natural because it is an ordinary everyday matter. And we are under no
more obligation to postulate supernatural control for the changing forms
in the life-history of a chick or a cat than we need to assume that
gravitation and the radiation of light demand immediate supernatural
direction. The embryology of no form is fully understood or described or
explained, but no intelligent person would be willing to assert that
because complete knowledge is lacking, it is unnatural for organic
transformation to take place during growth. Whatever may be the ultimate
origin and nature of the directing powers behind gravitation and
development and other phenomena, we have no concern with such matters
because they cannot be handled by scientific methods and one belief about
them is on the same plane with any other. Our task is to deal with the
everyday phenomena of life and the production of living species.

* * * * *

It is not necessary to go far afield to find an animal which will
introduce us to the general principles of embryology. In the present
instance as in the case of comparative anatomy almost any form will
disclose the meaning of development, for animate nature is uniform and
consistent in its methods of operation throughout its wide range. We shall
begin with the familiar frog which every one knows is a product of a
tadpole; passing on to the chick we will learn more facts that will enable
us to formulate the main principle of comparative embryology in definite
terms; we will then be prepared to extend our survey so as to include
somewhat less familiar facts and animals that are even more significant
than the first illustrations.

If we should visit a woodland pond in early spring, we would find
somewhere among the leaves and sticks in the water large masses of a clear
jellylike consistency enclosing hundreds of little black spheres about an
eighth of an inch in diameter. These are the egg masses and eggs of a
common frog. Watching them day by day we see the small one-celled egg
spheres divide into more and more numerous portions which are the
daughter-cells, destined to form by their products the many varied tissues
and organs of the developing larva and adult frog. After three or four
days the egg changes from its globular form into an oval or elliptical
mass, and from one end of this a small knob projects to become a flattened
waving tail a few days later. On the si