Smith.—Cy to logical Shidies in the Protococcales. III. 461 
more than one in ten thousand cells has more than one pyrenoid (Fig. 18). 
As the cell grows the pyrenoid increases in size (Figs. 1, n, 14), sometimes 
becoming irregular in shape (Figs. 3, 4, 5). Perhaps 2 per cent, of the cells 
show these irregularities. Irregular pyrenoids have been noted in Characium 
and Pediastrnm and the possibility of their being division stages discussed. 
If irregular pyrenoids are to be interpreted as division stages, cells with 
two pyrenoids should be as numerous as those with irregular ones, but since 
we find that irregular pyrenoids are a hundred times as numerous in 
T. minimum as cells with two pyrenoids, further evidence is furnished that 
such irregular pyrenoids are not division stages. Irregular pyrenoids can 
also be seen in living cells, so that they cannot be considered artifacts of 
fixation. There are also starch plates forming a hollow sphere which 
encloses the pyrenoid, but these are very delicate and are only noted in the 
most favourable preparations. In rare instances e stroma’ starch plates, 
a structure which has been discussed elsewhere ( 4 ), were observed (Fig. 13). 
There is no definite correlation between the size of the cell and the 
number of nuclei; since cells of the same size may have one (Fig. 7 ) or four 
(Fig. 13). Nuclear divisions are always simultaneous in multinucleate 
cells and all mitotic figures in such a cell are at the same stage of the pro¬ 
cess (Figs. 7, 11). Autospore formation may begin in cells containing only 
two nuclei, or it may not begin until eight nuclei are present, but in the vast 
majority of cases the process commences in tetranucleate cells. This 
variation in number of nuclei at the beginning of reproduction has been 
noted in a number of algae and is connected with the nutrition of the cell, 
the better nourished ones containing the larger number of nuclei. Stages 
in autospore formation are more abundant in material fixed during the 
night, but it is not especially marked at any one time. 
The following account is based upon the behaviour of those cells con¬ 
taining four nuclei when autospore formation commences. The first step is 
a transverse cleavage of the cytoplasm into two equal parts. The line of 
cleavage is always at right angles to the cell wall and, when the cell is 
oblong in cross-section, in the shorter diameter. This cleavage is probably 
very rapid, since no stages were found which showed it in progress. It is 
similar to that of the other Protococcales which I have studied— Tetra- 
desmus ( 3 ), Scenedesmus ( 4 ), Characium ( 5 ), and Pediastrnm (6); the 
pyrenoid remaining unchanged in one of the two daughter-cells (Fig. 31). 
Since this lack of pyrenoid division has been noted in five members of the 
order, belonging to four families, it seems a reasonable assumption that it is 
a generally distributed phenomenon in the Protococcales. It must not be 
inferred that this generalization is true for all Chlorophyceae, for there are 
several well-authenticated cases, especially in the Zygnemales, where the 
pyrenoid divides before cytokinesis. 
There is both direct and indirect evidence that karyokinesis follows the 
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