tort] . LUTMAN—CLOSTERIUM 413 
occur on both the concave and convex sides, but I have found them 
in these species to be practically all on the convex side. 
The movement of the nuclei is apparently amoeboid, comparable 
to that of the male nucleus of the flowering plants in its progress 
toward the egg nucleus in the embryo sac. In the present case, 
however, it seems conceivable that the pushing outward of material 
into the new end of either daughter Closterium, combined with the 
change of shape which that end undergoes as soon as the new wall 
is formed, may assist to some extent in the movement of the 
nuclei. It will not explain it entirely, however, and we are forced 
to assume some stimulus that induces the nucleus to move to its 
new position. As long as the nucleus is at the end or along the 
side of the chromatophore, the two sides of the nucleus are exposed 
unequally to the food supply, and while the active streaming move- 
ments in the peripheral granular layer would partly equalize the 
deficiency, the side that is next the chromatophore is in touch 
with the soluble foods that are available there in greater quantity 
than at the side next the granular layer. The only way in which 
the nucleus can supply itself equally with food over its entire 
exterior surface is to imbed itself near the center of the chromato- 
phore, and this is the position of course which it finally assumes. 
Other stimuli may of course also be in play. 
Soon after the new cross-wall is put in at the middle of the 
desmid, the new end begins to round out, but the two organisms 
hang together for quite a time, with only a very slight connection 
figs. 5, 6, 10). The connection finally breaks and the individuals 
Separate before the new halves are at all symmetrical with the old 
Ones (figs. 8, 12). The granular material which formed a band 
across the cell now makes a thin cap for each new half (fig. 6), the 
material in it being continuous with the plasma membrane and 
forming a conspicuous example of Mo t’s embryonic substance. 
The change of form of this new end, resulting in an organism with 
symmetrical chromatophores, and the formation of the end vacuole 
in the granular portion follow slowly (figs. 8, 11, 12, I 3)... Fhe 
Process as we can see it, while superficially an apparently very 
simple one, is extremely difficult of explanation in terms of physics 
