ere the two
combining cells are apparently of precisely similar nature and
structure. I say apparently, because if they are really alike it is
difficult to see what is gained by the union.

Conjugation occurs in algae and fungi. A typical case is that of
Spirogyra. This is an alga with its cells in long filaments. Two
contiguous cells of two parallel filaments push each a little projection
from its cell-wall toward the other. When these meet, the protoplasm of
each of the two cells contracts, and assumes an elliptical form--it
undergoes rejuvenescence. Next an opening forms where the two cells are
in contact, and the contents of one cell pass over into the other, where
the two protoplasmic bodies coalesce, contract, and develop a cell-wall.
The zygospore thus formed germinates after a long period and forms a new
filament of cells.

Another example of conjugation is that of Pandorina, an alga allied to
the well-known volvox. Here the conjugating cells swim free in water;
they have no cell-wall, and move actively by cilia. Two out of a number
approach, coalesce, contract, and secrete a cell-wall. After a long
period of rest, this zygospore allows the whole of its contents to
escape as a swarm-spore, which after a time secretes a gelatinous wall,
and by division reproduces the sixteen-celled family.

We now come to fertilization, where the uniting cells are of two kinds.

The simplest case is that of Vaucheria, an alga. Here the vegetative
filament puts out two protuberances, which become shut off from the body
of the filament by partitions. The protoplasm in one of these
protuberances arranges itself into a round mass--the oosphere or female
cell. The protoplasm of the other protuberance divides into many small
masses, furnished with cilia, the spermatozoids or male cells. Each
protuberance bursts, and some of the spermatozoids come in contact with
and are absorbed by the oosphere, which then secretes a cell-wall, and
after a time germinates.

The most advanced type of fertilization is that of angiosperms.

In them there are these differences from the above process: the contents
of the male cell, represented by the pollen, are not differentiated into
spermatozoids, and there is no actual contact between the contents of
the pollen tube and the germinal vesicle, but according to Strashurger,
there is a transference of the substance of the nucleus of the pollen
cell to that of the germinal vesicle by osmose. The coales