690 McAllister . — Nuclear Division in Tetraspora lubrica. 
halves with a fairly broad cleft between them ’. Not uncommonly the 
pyrenoid seems to be a solid mass with no plane of cleavage. Frequently 
they appear as rounded bodies (Figs. 13 and 14). These may in part of 
course be the flattened pyrenoids as seen from the side. Many of these 
rounded pyrenoids have undergone peripheral cleavage to form a number 
of smaller irregular starch masses which are perfectly distinct from one 
another (Fig. 31). They stain a uniform blue colour and show no dif- 
ferentiated central part such as is so well known for Spirogyra and has been 
described as a saffranin staining region by Timberlake (47) for Hydrodictyon , 
and by Lutman (32) for Closterium. They are, apparently, more like the 
pyrenoids of Cladophora , Oedogonium , or Rhizoclonium in which, according 
to Timberlake, c in some instances the entire pyrenoid is converted into 
starch without previous cleavage.’ As mentioned above, the protoplasm in 
which the pyrenoids lie cannot be distinguished in texture and staining 
reaction from that of other parts of the cell, though a narrow unstained zone 
is usually present immediately surrounding the pyrenoid. 
Discussion. 
It will be seen from the foregoing that the general details of the 
nuclear structure and mitosis in Tetraspora and in other Chlorophyceae 
thus far investigated, are essentially the same. As far as can be determined 
from the minute nuclei of Tetraspora the structure of the resting nuclei, and 
the conduct of the chromatin in spireme formation, the origin and develop- 
ment of the spindle and the mode of the formation of the cell-plate are pro- 
cesses the same as in the Angiosperms. A striking uniformity thus exists 
throughout the green plants in the phenomena of nuclear division. 
Accounts of the presence of centrosomes or centrospheres in certain 
cells of the Liverworts have encouraged the expectation that such centres 
should be common in the Chlorophyceae. Reports of great variation in the 
polar organization in the nuclei of the Protozoa, on the other hand, have 
contributed to this belief, — based on the view that certain Protozoans and 
the Chlorophyceae have probably arisen from a common ancestor. In- 
vestigations upon the mitosis in the green Algae have not sustained this 
expectation. The presence of centrosomes or centrospheres has thus far 
not been satisfactorily demonstrated in cells of this group of plants. It 
must be admitted, however, that the structures reported for Valonia (17), 
Derbesia (13), and Hydrodictyon (48, 59) strongly suggest centrosomes, and 
further investigations may prove them to be such. Nevertheless, they are 
clearly not such permanent centres as have been described by Harper (21) 
for Phyllactinia , as well as by others for various plant and animal cells. 
The presence of intranuclear spindles in Derbesia , Valonia , and possibly 
in Hydrodictyon is suggestive of a permanent centre. Still, mitosis in 
