Smith . — - Cytological Studies in the Protococcales. II. 477 
without cleavage planes are never found in preparations having angular 
protoplasts, but are always found in material fixed at a time after the 
cleavage into uninucleate protoplasts has been completed. Even stronger 
evidence upon this question is afforded by the condition of the cell contents, 
in that the nuclei are dense and irregular after the cleavage furrows have 
disappeared, and pyrenoids are never found in these cells (Fig. 19 and 
Text-fig. 4). On the other hand, cells are sometimes found in which there 
has been the preparatory division of the nuclei, but in which for some reason 
there has been no cleavage. These cells are easily distinguished from the 
stage just described by the less dense, spherical nuclei and the presence of 
a pyrenoid (Fig. 22, a). 
The next noticeable change is the reappearance of the cleavage planes. 
These look in section like very fine lines, and at the time that they reappear 
the protoplasts are fully rounded up. In a few instances the planes made 
their appearance at one end only of the cell (Fig. 21), but in the majority of 
cases the process of reappearance seems to be simultaneous in all parts 
of the cell (Fig. 22, b). At this time the nuclei are even more elongate and 
dense than at any previous stage in the cleavage process, and the chromatin 
granules can only be distinguished with the greatest difficulty. These 
rounded, uninucleate protoplasts are the zoospores. 
After the reappearance of the cleavage planes the protoplasts separate 
slightly from one another, the nuclei remaining flattened (Fig. 20). I am 
in doubt concerning the very last developmental stages, since in the fixed 
preparations it is impossible to say whether a definite cavity contains 
zoospores or zoospores that have germinated into cells. In certain instances 
there is no doubt that the zoospores have germinated into cells within the 
mother-cell wall (Fig. 24). On account of these difficulties it is impossible 
to determine whether the nuclei, which are very much flattened when the 
cleavage planes reappear, become rounded up in the zoospore stage or after 
the zoospores have germinated to form cells. Nor am I certain at what 
precise step the pyrenoids, which are always found in the youngest cells, 
reappear. 
Summary. 
* 
The youngest cells of Pediastrum Boryanum are uninucleate, each 
containing one, rarely two or three, pyrenoids. Mature cells contain four or 
eight nuclei and one to three pyrenoids. The nuclei increase in number by 
simultaneous division so that the number is always a multiple of two. 
In structure the resting nucleus differs but little from the nucleus of the 
higher plants. 
The pyrenoids are homogeneous in structure and surrounded by curved 
starch plates. 
