ws the two chromosomes retained in the egg
forming the female pronucleus. The centrosome has disappeared.]

Previously to this process the other sexual cell, the _spermatozoon_, or
male reproductive cell, has been undergoing a somewhat similar process.
This is also a true cell (Fig. 34, _mc_), although it is of a decidedly
smaller size than the egg and of a very different shape. It contains
cell substance, a nucleus with chromosomes, and a centrosome, the number
of chromosomes, as shown later, being however only half that normal for
the ordinary cells of the animals. The study of the development of the
spermatozoon shows that it has come from cells which contained the
normal number of four, but that this number has been reduced to one half
by a process which is equivalent to that which we have just noticed in
the egg. Thus it comes about that each of the sexual elements, the egg
and the spermatozoon, now contains one half the normal number of
chromosomes.

[Illustration: FIG. 36--The egg centrosomes have changed their position.
The male cell with its centrosome remains inactive until the stage
represented in Fig. 42.]

[Illustration: FIG. 37--Beginning of the first division for removing
superfluous chromosomes.]

Now by some mechanical means these two reproductive cells are brought in
contact with each other, shown in Fig. 34, and as soon as they are
brought into each other's vicinity the male cell buries its head in the
body of the egg. The tail by which it has been moving is cast off, and
the head containing the chromosomes and the centrosome enters the egg,
forming what is called the male pronucleus (Figs. 35-38, _mn_). This
entrance of the male cell occurs either before the formation of the
polar cells of the egg or afterward. If, however, it takes place before,
the male pronucleus simply remains dormant in the egg while the polar
cells are being protruded, and not until after that process is concluded
does it begin again to show signs of activity which result in the cell
union.

The further steps in this process appear to be controlled by the
centrosome, although it is not quite certain whence this centrosome is
derived. Originally, as we have seen, the egg contained a centrosome,
and the male cell has also brought a second into the egg (Fig. 35,
_ce_). In some cases, and this is true for the worm we are describing,
it is certain that the egg centrosome disappears while that of the
spermatozoon is retained