302 Smith. — A Note on Conjugation in Zygnema. 
Zygnema stellinum : cells of filament sub-cylindrical 30-35 (33) \x in 
diameter, 70-80 /ot in length. The cell-wall is thin cellulose, with no signs 
of lamellation even when treated with caustic potash. The cytoplasm is 
somewhat vacuolated, with an apparently denser median strand (zugon), in 
which the nucleus is suspended. The nucleus is small (3 /x), sub-spherical, 
with well-marked membrane and nucleolus. No details of nuclear division 
were observed. In the vegetative cell there are two elaborate chloroplasts, 
suspended in a small amount of cytoplasm in the centre of the cell. Each 
plastid contains a single large (3 /x) pyrenoid, which is surrounded by 
a more or less diffuse starch zone. The exact method of starch-formation 
was not determined. The plastid is never apparently seen side-on, as it is 
in Mougeotia , so that it would seem to be a radially organized body. It 
appears (in the fresh material) as a complex mass of much-dissected fronds, 
radiating from and incurved over the central pyrenoid (see Plate XII, Fig. 1). 
Text-fig. i. Zygnetna stellinum , showing scalariform type of conjugation. ( x 500 times.) 
In the fixed material the dissection was not quite so marked, due possibly 
to shrinkage during manipulation. 
Conjugation. The material at hand showed abundance of zygospores, 
but no earlier stages in the process of conjugation. Conjugation was by 
the scalariform method, the zygospore being lodged in one or other of the 
conjugating cells, which is not inflated or distorted in any way (Text- 
fig. 1). ‘ Cross ’-conjugation was not observed, but there were numerous 
cases of differentiation of sex in a single filament, as has been frequently 
reported in Spirogyra (Cunningham ( 1 )). 
The young zygospore is 30-33 /x in diameter, and approximately 
spherical. The wall is thin (1*5— 2 /x), and not yet cuticularized : usually 
there are four inwardly-directed points on the inner surface. The four 
chloroplasts derived from the two conjugants remain quite distinct, and 
retain their characteristic complex shape as seen in the vegetative cell. 
They are commonly arranged in a tetrahedral fashion (see Plate XII, 
Fig. 2). 
