ilaginous coat of the ovum; a prominence is rising from the
surface of the ovum towards a spermatozooen; B, they have almost met;
C, they have met; D, the spermatozooen enters the ovum through a
distinct opening; H, the entire ovum, showing extruded polar bodies
on its upper surface, and the moving together of the male and female
pronuclei; E, F, G, meeting and coalescence of the pronuclei.]

The sperm-cell, or spermatozooen, is seen in the act of penetrating the
ovum. In the first figure it has already pierced the mucilaginous coat
of the ovum, the limit of which is represented by a line through which
the tail of the spermatozooen is passing: the head of the spermatozooen is
just entering the ovum proper. It may be noted that, in the case of many
animals, the general protoplasm of the ovum becomes aware, so to speak,
of the approach of a spermatozooen, and sends up a process to meet it.
(Fig. 35, A, B, C.) Several--or even many--spermatozoa may thus enter
the coat of the ovum; but normally only one proceeds further, or right
into the substance of the ovum, for the purpose of effecting
fertilization. This spermatozooen, as soon as it enters the periphery of
the yolk, or cell-substance proper, sets up a series of remarkable
phenomena. First, its own head rapidly increases in size, and takes on
the appearance of a cell-nucleus: this is called the male pronucleus. At
the same time its tail begins to disappear, and the enlarged head
proceeds to make its way directly towards the nucleus of the ovum which,
as before stated, is now called the female pronucleus. The latter in its
turn moves towards the former, and when the two meet they fuse into one
mass, forming a new nucleus. Before the two actually meet, the
spermatozooen has lost its tail altogether; and it is noteworthy that
during its passage through the protoplasmic cell-contents of the ovum,
it appears to exercise upon this protoplasm an attractive influence; for
the granules of the latter in its vicinity dispose themselves around it
in radiating lines. All these various phenomena are depicted in the
above wood-cuts. (Figs. 34, 35.)

Fertilization having been thus effected by fusion of the male and female
pronuclei into a single (or new) nucleus, this latter body proceeds to
exhibit complicated processes of karyokinesis, which, as before shown,
are preliminary to nuclear division in the case of egg-cells. Indeed the
karyokinetic process may begin in