s of higher animals--muscular, connective, glandular,
etc. And by tissues we mean groups of cells modified in form and
structure for the performance of a special work or function. The
protozoa developed the cell for all time to come, the coelenterata
developed the tissues which still compose our bodies. But they had
them mainly in a diffuse form. A sort of digestive and reproductive
system they did possess. But the work of arranging these tissues and
condensing them into compact organs was to be done by the next
higher group, the worms.

Let us now take a glance at certain stages of embryonic development
which correspond to these earliest ancestral forms. We should expect
some such correspondence from the fact already stated that the
embryonic development of the individual is a brief recapitulation of
the ancestral development of the species or larger group. The egg of
the lowest vertebrate, amphioxus, shows these changes in a simple
and apparently primitive form.

[Illustration: 3. IMMATURE EGG-SHELL FROM OVARY OF ECHINODERM.
HATSCHEK, FROM HERTWIG.]

The fertilized egg of any animal consists of a single cell, a little
mass of protoplasm containing a nucleus and surrounded by a
structureless membrane. The egg is globular. The nucleus undergoes
certain very peculiar, still but little understood, changes and
divides into two. The protoplasm also soon divides into two masses
clustering each around its own nucleus. The plane of division will
be marked around the outside by a circular furrow, but the cells
will still remain united by a large part of the membrane which
bounds their adjacent, newly formed, internal faces.

Let us suppose that the egg lay so that the first plane of division
was vertical and extending north and south. Each cell or half of the
egg will divide into two precisely as before. The new plane of
division will be vertical, but extending east and west. Each plane
passes through the centre of the egg, and the four cells are of the
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