e rapid growth and reproduction
of certain cells of the entoderm which have migrated, so to speak,
into this new position. In higher forms it becomes of continually
greater importance, until finally nearly all the organs of the body
develop from it. In our bodies only the lining of the mid-intestine
and of its glands has arisen from the entoderm. And only the
epidermis, or outer layer of our skin, and the nervous system and
parts of our sense-organs have arisen from the ectoderm. But our
mid-intestine is still the greatly elongated archenteron of the
gastrula.

We may therefore compare the hydra or gastrula to a little portion
of the lining of the human mid-intestine covered with a little flake
of epidermis. This much the hydra has attained. But our bones and
muscles and blood-vessels all come from the mesoderm by folding,
plaiting, and channelling, and division of labor resulting in
differentiation of structure. Of all true mesodermal structures the
hydra has actually none, but in the ectodermal and entodermal cells
he has the potentiality of them all. We must now try to discover how
these potentialities became actualities in higher forms.

The third stage in our ancestral series is the turbellarian. This is
a little, flat, oval worm, varying greatly in size in different
species, and found both in fresh and salt water. Some would deny
that this worm belonged in our series at all. But, while doubtless
considerably modified, it has still retained many characteristics
almost certainly possessed by our primitive bilateral ancestor. The
different parts of hydra were arranged like those of most flowers,
around one main vertical axis; it was thus radiate in structure,
having neither front nor rear, right nor left side. But our little
turbellaria, while still without a head, has one end which goes
first and can be called the front end. The upper or dorsal surface
is usually more colored with pigment cells than the lower or ventral
surface, on which is the mouth. It has also a right and left side.
It is thus bilateral.

The gastraea swam by cilia, little eyelash-like processes which urge
the animal forward like a myriad of microscopic oars. In our bodies
they are sometimes used to keep up a current, _e.g._, to remove
foreign particles from the lungs. The turbellaria is still covered
with cilia, probably an inheritance from the gastraea; for, while in
smaller forms they may still be the principal means of locomotion,
in lar