same form and size, like much-rounded quarters of an orange. The
third plane will lie horizontal or equatorial, and will divide each
of these quarters into an upper and lower octant. The cells keep on
dividing rapidly, the eight form sixteen, then thirty-two, etc. The
sharp angle by which the cells met at the centre has become rounded
off, and has left a little space, the segmentation cavity, filled
with fluid in the middle of the embryo. The cells continue to press
or 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.