be crowded away from the centre and form a layer one cell deep on
the surface of the sphere.

This embryo, resembling a hollow rubber ball filled with fluid, is
called a blastosphere. It corresponds in structure with the fully
developed volvox, except, of course, in lacking reproductive cells.

[Illustration: 4. GASTRULA. HATSCHEK, FROM HERTWIG.
Outer layer is the ectoderm; inner layer, the entoderm; internal
cavity, the archenteron; mouth of cavity, blastopore.]

If the rubber ball has a hole in it so that I can squeeze out the
water, I can thrust the one-half into the other, and change the ball
into a double-walled cup. A similar change takes place in the
embryo. The cells of the lower half of the blastosphere are slightly
larger than those of the upper half. This lower hemisphere flattens
and then thrusts itself, or is invaginated, into the upper
hemisphere of smaller cells and forms its lining. This cup-shaped
embryo is called the gastrula. The cup deepens somewhat and becomes
ovoid. Take a boiled egg, make a hole in the smaller end and remove
the yolk, and you have a passable model of a gastrula. The shell
corresponds to the ectoderm or outer layer of smaller cells; the
layer of "white" represents the entoderm or lining of larger cells.
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 gastraea, 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 th