Smith. — Cytological Studies in the Protococcales. II. 469 
a sharply defined nuclear membrane. The chromatin and the linin content 
vary, however, at different stages in the life history. In very young cells 
the chromatin granules are pronounced and near the nuclear membrane, but 
the linin threads can be distinguished only with difficulty (PI. XII, Fig. 1). 
At this time the nucleolus is less conspicuous than in the later stages. As 
the cell becomes older the nucleolus and the linin threads grow more promi- 
nent, whereas the chromatin granules become less conspicuous, but never so 
indistinct as not to be recognizable in good preparations (Fig. 31, 22). 
Although many of the details are lacking, there is sufficient evidence to 
show that the nuclear divisions are mitotic. In the equatorial plate stage a 
distinctly bipolar spindle can be recognized. The chromosomes are arranged 
Text-fig. i. Young colonies showing that the nucleus and pyrenoid of a cell are not definitely 
located with respect to the margin of the colony. ( x 2,000.) 
in a dense group when observed from the side (Fig. 2), but in polar views 
they appear much more distinctly (Figs. 4, 5). Although Timberlake ( 17 ) 
describes a small, spherical, densely staining body at each pole of the 
spindle in the closely related Hydrodictyon , I have not observed anything 
of the sort in Pediastrum. Since no cells were found in which some of the 
nuclei were dividing and some were not, it may be safely assumed that the 
nuclei within a cell divide simultaneously. 
The cytoplasm of the youngest cells is granular, but a little later 
scattered vacuoles appear (Fig. 3). These vacuoles never become a 
conspicuous feature, however, and do not fuse to form a large central 
vacuole. As the cell becomes still older they frequently disappear, so that 
the mature cell again seems to be filled with a dense granular cytoplasm 
(Figs. 5, 6). The cell grows to a considerable size before the first nuclear 
