vin
other species parallel or crossing each other in different ways.
Out of the contiguous sides each cell projects a short cylindrical
“ copulation process,” rounded at the ends. Each advances towards
the other until they meet and coalesce. In some species
the process emerges in the first instance from a gaping fissure
in the original cell wall; in others there is no cracking of the
cell or m-oss fissure of the membrane. The «copulation processes
” are in all cases surrounded by a tender membrane ; at
the free end the membrane is thicker. The contents of the
“ process ” consist of a colourless plasma of watery fluidity, in
which separate granules are suspended.
^ Both “ copulation processes ” swell, apparently at the same
time, into wide bladders, of which one quickly attains a semicircular
form, touching the other inwardly with a flat surface.
The parting wall which separates them is immediately dissipated.
The primordial membranes are melted into one, and a continually
swelling middle space is formed, uniting the two cells. At this
time the primordial membrane has been loosened from the cell
membrane of the four half-cells, and gradually retreats towards
the middle. A t the first, only protoplasm and colourless fluid
pass over from the old cells into the middle space, then follow
the cell-kernels from both sides ; and, lastly, the chlorophyll,
with the last ends of the primordial utricle. In the cases
observed by De Bary there elapsed 15, 20 to 40 minutes from
the first swelling of the processes until the complete absorption
of the primordial membrane from the four half-cells.
^ From this point forward there are slight variations, in
different species, in the development of the zygospore from the
swollen central space lying between the old semi-cells. In
Cosmarium botrytis the middle space is, during its swelling, surrounded
by a tolerably thick, but uncommonly tender, membrane,
having the appearance of a homogeneous. gelatinous
wall. Inside the mucous bladder, which is continually becoming
paler, the primordial membrane contracts itself to a
regular globular form. Already, after 10-15 minutes, it
possesses a visibly tender, almost completely smooth, cellulose
skin. After about two hours the first intimations of the later
spikes of the zygospore are visible as small projections of the
cell wall. The successive formation of the three skins which
clothe the ripe zygospore then follow.
The difference between the forms of copulation consists in
this, that the middle space in Closterium parvulum is surrounded
by a permanent cellulose skin ; in the others by a
mucous membrane, which often becomes invisible. Most of the
conditions described by Ealfs range themselves between these
two forms, or are transitions towards them.
The structure of the mature zygospore corresponds in the
Desmidieæ with that of Zygnema, &c. There exist three
cuticles in succession from the outside to the inside ; the
outer cuticle of cellulose ; the middle cuticle, generally, though
not always, tinged, and often very dark-brown ; and a colourless
inner cuticle exhibiting the reaction of cellular tissue.
The ripe zygospores of Cosmarium, Euastrum, and allied
species are known to have on their outer surface prickles and
projections of various shapes. In extreme youth the surface is
smooth. Soon we see numerous projections thrown up, until
the prominences assume the characteristic form and size of the
mature prickles of the species. Their membrane thickens
itself, while it deposits new cellulose sheaths from the outward
end towards the inside. The spines, therefore, soon
become solid bodies. In the contents of the ripe zygospore
has been found chlorophyll and starch, the latter partly replaced
by fat.
Observation of the process of germination, or the development
of new individuals from the zygospore, seems to be rare. Wolle
summarizes the result of De Bary’s observations thus—“ The
envelope of the matrix or zygospore is primarily thin and smooth,
but by degrees it acquires increased thickness, and in the Cosma-
riums usually a granular, tuberculated, or, more frequently, a
spinous surface, the spines being sometimes simple, but commonly
forked at the extremities. The next step, so difficult to be traced,
is the opening of the wall of the zygospore, setting free small
spheres of sarcode. As they issue they enlarge, and acquire a gelatinous
or thin membranous wall. Thewall thickens, and the sphere
enlarges, the contents constrict first in one direction, and then
transversely to the plane of the first incision. These parts