MESOTANIUM, AND SPIROTANIA. 215 
characteristic we have to some extent an analogy in Vaucheria, 
whose fertilised oospore does not develop daughter-cells, each 
to give origin to so many new individuals, but grows at once 
into a single new plant, unicellular, of course, like its parent. 
But, notwithstanding all these so varied, more or less com- 
plex conditions, it may, perhaps, be still urged that, after all, 
such conjugation is but the union of the contents of two 
morphologically equivalent cells. 
To this objection the conditions in the genus Sirogonium 
seem to afford a valid answer. 
Two ordinary joints of a filament in Sirogonium mutually 
send out short processes, as in Spirogyra, which become 
united ; thereupon there ensues the formation of a septum 
(similarly to that of the vegetative cell) in each of these 
united cells. In one this septum, however, unlike the septum 
of a simply vegetating cell, divides the mother-cell into two 
very unequal daughter-cells, the larger of which becomes ex- 
ternally expanded. This larger expanded daughter-cell is that 
one which bears the extension joining it to the other opposite 
conjugating cell, and is constantly the receiving cell—that is, 
the one ultimately to contain the zygospore. Its sister-cell 
—the smaller one—remains sterile, being shut off from par- 
cipating in the conjugation. The other opposite conjigating 
cell also becomes divided by a septum into two daughter-cells, 
a short and a long one; but in this instance it is the shorter 
daughter-cell to which the extension joiming it to the other 
conjugating cell belongs, and this cell is in conjugation con- 
stantly the giving cell; its sister-cell—the larger one— 
remains sterile, being shut off from participating in the 
conjugation. The shorter or giving cell is itself sometimes 
again divided into two, one of which daughter-cells is shut off 
from participating in the conjugation. Speedily the contents 
of the two connected cells become increased in quantity and 
density, so as more nearly to fill the cells, quite unlike the 
sparse, pale (yellowish-green), and narrow irregular bands 
formed by the endochrome of the simply vegetative cells. 
The contents of the two conjugated cells now become 
contracted from the cell-wall; the intervening septum 
of the tubular imosculated connecting processes becomes 
resorbed ; the contents of the smaller of the two passes 
over, as in Spirogyra, and becomes formed, within that of 
the other, incorporating with its contents, into a zygospore. 
Here, then, is a conjugation between two cells of not mor- 
phologically equivalent, but which are evidently specialised 
structures. Here the giving and receiving cells seem to be 
as morphologically distinct as in Gdogonium curvum, in 
