The space occupied by the yolk corresponds to the archenteron or
primitive digestive cavity; and the opening at the end to the
primitive mouth or blastopore. Ectoderm and entoderm unite around
the mouth. Both the blastosphere and gastrula often swim freely by
flagella.

You can hardly have failed to notice how closely the gastrula
corresponds to a hydra, and many facts lead us to believe that the
still earlier ancestor of the hydra was free swimming, and that the
tentacles are a later development correlated with its adult sessile
life. Yet we must not forget that the hydra is even now not quite
sessile, it moves somewhat. And our ancestor was almost certainly a
free swimming gastræa, or hypothetical form corresponding in form
and structure to the gastrula. The ancestor of man never settled
down lazily into a sessile life.

But how is an adult worm or vertebrate formed out of such a
gastrula? To answer this would require a course of lectures on
embryology. But certain changes interest us. Between the ectoderm
and entoderm of the gastrula, in the space occupied by the
supporting membrane of hydra, a new layer of cells, the mesoderm,
appears. This has been produced by the 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