globule; the segmentation-cells are loosened, and all rise to the
surface. There they are flattened by mutual pressure, and assume the
shape of truncated pyramids, and arrange themselves side by side in
one regular layer (Figures F, G). This layer of cells is called the
germinal membrane (or blastoderm); the homogeneous cells which compose
its simple structure are called blastodermic cells; and the whole
hollow sphere, the walls of which are made of the preceding, is called
the blastula or blastosphere.* (* The blastula of the lower animals
must not be confused with the very different blastula of the mammal,
which is properly called the gastrocystis or blastocystis. This
cenogenetic gastrocystis and the palingenetic blastula are sometimes
very wrongly comprised under the common name of blastula or vesicula
blastodermica.)

In the case of our coral, and of many other lower forms of animal
life, the young embryo begins at once to move independently and swim
about in the water. A fine, long, thread-like process, a sort of whip
or lash, grows out of each blastodermic cell, and this independently
executes vibratory movements, slow at first, but quicker after a time
(Figure F). In this way each blastodermic cell becomes a ciliated
cell. The combined force of all these vibrating lashes causes the
whole blastula to move about in a rotatory fashion. In many other
animals, especially those in which the embryo develops within enclosed
membranes, the ciliated cells are only formed at a later stage, or
even not formed at all. The blastosphere may grow and expand by the
blastodermic cells (at the surface of the sphere) dividing and
increasing, and more fluid is secreted in the internal cavity. There
are still to-day some organisms that remain throughout life at the
structural stage of the blastula--hollow vesicles that swim about by a
ciliary movement in the water, the wall of which is composed of a