628 Timber lake. — Starch- Formation in 
a fashion as to leave an undifferentiated portion on either 
side of the starch-grain (Figs. 10 and 15). In the latter 
figure the constricted appearance of the pyrenoid in the region 
of starch-formation is noteworthy. The explanation of this 
fact is not evident, but it may be due simply to the shape 
of the grains earlier formed, an hypothesis that seems to be 
supported by the shape of the grains on either side of the 
pyrenoid. Fig. 11 is quite striking in this respect, while 
Fig. 1 2 shows no such clear relation of the shape of the starch- 
grains to that of the pyrenoids. Note also that in Figs. 11 
and 12 there is no evidence of starch-formation in the con- 
stricted region. 
Whether the pair of pyrenoids in the central part of Fig. 6 
is the result of the division of a single pyrenoid is not clear. 
The peculiar shape of the starch-grain lying between the two 
pyrenoids is uncommon, and may possibly be the result of 
a fusion of the parts of two closely adjacent pyrenoids that 
are being metamorphized into starch. But earlier stages 
indicating such a fusion have not been observed, and it is 
not impossible that the starch-grain simply represents a 
middle portion of the original single pyrenoid after it had 
the constricted appearance, as a result of the formation of the 
older grains on either side. 
It is, I think, sufficiently clear that there may be a division 
of the pyrenoid due to a transformation of a certain part of it 
into starch. Whether all the cases of division of the pyrenoid 
that have been described by other observers are to be ex- 
plained in this manner is an interesting question that needs 
further careful investigation. The apparent time-relations 
of the division of the pyrenoids in Spirogyra and Zygnema 
(in the one case immediately preceding and in the other im- 
mediately following the division of the cell), as described by 
Strasburger and Chmielewski, would seem to have little rela- 
tion to the process as described above for Hydrodictyon. 
Still, a single pyrenoid in the condition shown in Figs. 10 
or 18 might well suggest, if taken alone, a reproduction of 
pyrenoids rather than starch-formation. That the pyrenoids 
