des
into two parts, which remain for a time connected by delicate
threads (Fig. 18, _B_), that finally disappear. At first no nucleoli
are present in the daughter nuclei, but they appear before the
division is complete.

New filaments are formed by the breaking up of the old ones, this
sometimes being very rapid. As the cells break apart, the free ends
bulge strongly, showing the pressure exerted upon the cell wall by
the contents (Fig. 18, _A_).

Spores like those of _OEdogonium_ are formed, but the process is
somewhat different. It occurs in most species late in the spring, but
may sometimes be met with at other times. The masses of fruiting
plants usually appear brownish colored. If spores have been formed
they can, in the larger species at least, be seen with a hand lens,
appearing as rows of dark-colored specks.

Two filaments lying side by side send out protuberances of the cell
wall that grow toward each other until they touch (Fig. 18, _D_). At
the point of contact, the wall is absorbed, forming a continuous
channel from one cell to the other. This process usually takes place
in all the cells of the two filaments, so that the two filaments,
connected by tubes at regular intervals, have the form of a ladder.

In some species adjoining cells of the same filament become
connected, the tubes being formed at the end of the cells (Fig. 18,
_H_), and the cell in which the spore is formed enlarges.

Soon after the channel is completed, the contents of one cell flow
slowly through it into the neighboring cell, and the protoplasm of
the two fuses into one mass. (The union of the nuclei has also been
observed.) The young spore thus formed contracts somewhat, becoming
oval in form, and soon secretes a thick wall, colorless at first,
but afterwards becoming brown and more or less opaque. The
chlorophyll bands, although much crowded, are at first
distinguishable, but later lose the chlorophyll, and become
unrecognizable. Like the resting spores of _OEdogonium_ these require
a long period of rest before germinating.

[Illustration: FIG. 19.--Forms of _Zygnemaceae_. _A_, _Zygnema_. _B_,
_C_, _D_, _Mesocarpus_. All x 150.]

There are various genera of the pond scums, differing in the form of
the chloroplasts and also in the position of the spores. Of these may
be mentioned _Zygnema_ (Fig. 19, _A_), with two star-shaped
chloroplasts in each cell, and _Mesocarpus_ (Fig