Igtt] LUTMAN—CLOSTERIUM 421 
the final separation of the two halves seems to occur “under its 
direction.” In sections, the relation of the plasma membrane 
to the vacuole which is cutting the chromatophore in two is 
brought out very clearly (fig. 36). 
It is not possible to learn more of the movement of these nuclei 
in sections than it is in whole mounts. The process, however, 
seems to be some kind of an amoeboid one, judging from the 
changes in shape the nuclei undergo in the process. 
The formation of the daughter cell walls 
There seems to be a general agreement that in the Conjugatae 
the new cell wall arises by a growth inward from the old wall, 
in which the spindle fibers, unlike those of higher plants in which 
a cell plate is formed, take no part. The central spindle remains 
in place while the wall is growing (figs. 27, 29), but seems to take 
no visibly active part in its formation, although there is the proba- 
bility that its material may be used up to form the new plasma 
membrane. It is to be noted further that none of the fibers of the 
central spindle persist (figs. 33, 34) until they are cut in two by the 
ingrowth of the wall from the sides, as occurs in Spirogyra. 
The process of new wall-formation is essentially the same in 
the two species of Closterium studied as in Spirogyra. After the 
chromosomes have been drawn back to the two poles, the large 
central spindle, which previously had been very conspicuous, 
disappears, and in the later stages of the telophase all that can be 
Seen of it are a number of fibers extending through the region it 
formerly occupied (fig. 29). The cross-wall begins to grow in during 
Metaphase, the process starting at the periphery of the cell and 
seeming to be due to constriction of the plasma membrane. Pre- 
ceding and also accompanying this growth of the plasma membrane 
a layer of granular cytoplasm appears, in which the streaming 
Movements are very noticeable in the living specimens. The new 
Wall cuts across the spindle fairly at its center, being a third of the 
Way across by the time the fibers have disappeared. Connecting 
€ two nuclei is a granular band which, connecting with the 
Stowing plasma membrane, gives the appearance shown in 
8. 3 to the external view of the whole mount. By the time the 
in 
