otoplasm of the two cells fuses over the whole surface of contact,
and half the nucleus of the first cell penetrates the second cell,
while half the nucleus of the latter enters the first cell. After this
exchange the cells separate from each other and each exchanged half of
the nucleus fuses with the primitive half of the nucleus remaining in
the cell.

From this moment each cell continues to reproduce itself by fission,
as we have seen above. In another form, two cells meet and fuse
completely. Their nuclei become applied against each other and each
exchanges half its substance with the other as in the preceding case,
so that the final result is the same. In both cases the two conjugated
cells are identical, and one cannot call them male and female.

=Penetration of the Spermatozoid into the Egg.=--In all the higher
animals in which the germinal cells are of two kinds, male and female,
conjugation takes place in rather a different manner. Here, the female
cell or egg only reproduces itself exceptionally by parthenogenesis.
It usually contains no chromosomes and often too little chromatin, so
that it perishes when conjugation does not occur.

The spermatozoid swims by means of its tail to meet the egg. As soon
as it touches it it penetrates it and the coagulation which we have
mentioned is produced. This coagulation forms the _vitelline
membrane_, which prevents the entry of other spermatozoids. If, from
pathological causes the entry of several spermatozoids takes place,
there results, according to _Fol_, a double or triple monster.

In Fig. 11 on Plate II, we see the egg with its vitelline membrane and
nucleus, the chromatin network of which is marked in blue: _b_ shows
the protoplasm of the egg or _vitellus_; _a_ the vitelline membrane;
_d_ the spermatozoid which has just entered, and the nucleus of which,
composed chiefly of chromatin, is colored red, while its tail has
performed its task and is about to disappear. The letters _e_, _f_,
and _g_, show a spermatozoid which has arrived too late.

Before the head of the spermatozoid which has entered, appears a
centrosome (Fig. 12) which it brings to the egg with its small amount
of protoplasm, and around this centrosome rays form, as in the case of
cellular fission. At the same time a nuclear liquid arising from the
protoplasm of the egg becomes concentrated around the chromatin of the
spermatozoid, while the nucleus of the egg remains in place and does
not change