ugh to fertilise the egg. The sperm-cells swim in the water, and
are chemically attracted by the eggs. As there are so many sperms, one
of them is sure to reach each black egg-sphere. It drives its way into
the substance of the egg, making a minute hole in its surface; then
the protoplasm of the sperm fuses with the protoplasm of the egg, and
becomes intimately mixed with it. The egg-cell has a "nucleus," that
dense, peculiar, deep-lying, and well-marked "kernel" of its
protoplasm which all cells have. It is of essential importance in the
life and activity of the cell. The sperm-cell has also a "nucleus,"
and now (as has been carefully ascertained) the nucleus of the sperm
and the nucleus of the egg-cell unite and form one single nucleus. The
egg is thereupon said to be "fertilised"--that is to say, "rendered
fertile." It at once commences to move. Its surface ripples and
contracts and nips in deeply, so that the sphere is marked out into
two hemispheres. These are two "cells," or masses of protoplasm,
adhering to each other. Each is provided with its own distinct nucleus
or cell-kernel, for the first step in the division of the egg-sphere
is the division within it of its newly constituted nucleus into two,
each half consisting of nearly equal proportions of the mingled
substance of the sperm-nucleus and the egg-nucleus. The two first
cells or hemispheres again divide, and so the process goes on until
the little black egg has the appearance of a mulberry, each granule of
the berry being a cell provided with its own nucleus derived from the
original nucleus formed by the fusion of the nuclei of the paternal
and maternal cells. In the course of a day or two the division has
proceeded so far that the resulting "cells" are so small as to be
invisible with a hand-glass, and require one to use a high magnifying
power in order to distinguish them. And there are hundreds of them;
the whole mass of the "egg" within, as well as on the surface, has
divided into separate cells. They go on multiplying, take up water,
and nourish themselves on the granular nutritive matter present from
the first in the egg-cell. The little mass elongates, increases in
size, and gradually assumes the form of a young tadpole.

We see, then that the process of fertilisation consists in two things,
the latter of which necessitates the former, viz. in the breaking or
penetration of the surface of the egg-cell by the active sperm
filament and second in the