to be composed. That complicated molecular movement of the protoplasm
which we call "life" is, naturally, something quite different in this
stem-cell from what we find in the two parent-cells, from the
coalescence of which it has issued. THE LIFE OF THE STEM-CELL OR
CYTULA IS THE PRODUCT OR RESULTANT OF THE PATERNAL LIFE-MOVEMENT THAT
IS CONVEYED IN THE SPERMATOZOON AND THE MATERNAL LIFE-MOVEMENT THAT IS
CONTRIBUTED BY THE OVUM.

The admirable work done by recent observers has shown that the
individual development, in man and the other animals, commences with
the formation of a simple "stem-cell" of this character, and that this
then passes, by repeated segmentation (or cleavage), into a cluster of
cells, known as "the segmentation sphere" or "segmentation cells." The
process is most clearly observed in the ova of the echinoderms
(star-fishes, sea-urchins, etc.). The investigations of Oscar and
Richard Hertwig were chiefly directed to these. The main results may
be summed up as follows:--

Conception is preceded by certain preliminary changes, which are very
necessary--in fact, usually indispensable--for its occurrence. They
are comprised under the general heading of "Changes prior to
impregnation." In these the original nucleus of the ovum, the germinal
vesicle, is lost. Part of it is extruded, and part dissolved in the
cell contents; only a very small part of it is left to form the basis
of a fresh nucleus, the pronucleus femininus. It is the latter alone
that combines in conception with the invading nucleus of the
fertilising spermatozoon (the pronucleus masculinus).

The impregnation of the ovum commences with a decay of the germinal
vesicle, or the original nucleus of the ovum (Figure 1.8). We have