44 2 Acton. — Observations on the Cytology of the Chroococcaceae . 
either numerous small areas (Fig. 4), a few larger areas, or in some cases 
the whole central region seems to stain. 
The very regular shape of these areas seems to suggest that they are 
of the nature of vacuoles. If so, they are certainly not due to degeneration, 
for they are almost always found in dividing cells. Since the cells in which 
they occur always contain few metachromatin granules, while the majority 
of the cells in the collection are rich in metachromatin, it is probable that 
they are due to diffusion of the metachromatin when accumulation of reserve 
has reached its limit. 
With iodine-green-fuchsin these areas remain colourless or slightly 
blue, which is another argument in favour of diffusion of metachromatin 
(Fig- 5)- 
The cyanophycin granules were also found to be situated at the nodal 
points of the reticulum in the peripheral region of the cell. They are not 
situated in the absolute periphery of the cell. There is always a region 
immediately within the cell-wall in which the majority of the plasmatic 
microsomes stain only with cytoplasmic stain (Fig. 3). 
Division is brought about by the constriction of the cell into two 
approximately equal halves, caused by the ingrowth of the cell-wall. There 
is perhaps a tendency for the reticulum in the central region to be drawn 
out into more or less parallel lines, though the cross-connexions are not 
lost (Fig. 7). Again, the threads of the reticulum appear to stain more 
deeply in some cases in the central region, but this may only be accidental. 
V. Other Species of the Chroococcaceae. 
All the remaining species examined are characterized by a reticulum 
with plasmatic microsomes. The size of the mesh of the reticulum is 
approximately the same in the different species, so that in the smaller ones 
there are necessarily fewer granules (Figs. 12, 13, and 14). The other species 
of Chroococcus offer no features of particular interest. 
The species of Goniphosphaeria , Coelosphaerium , and Dactylococcopsis 
examined were too small for a clear interpretation of results, but do not 
appear to differ from the Chroococcus type- 
Aphanothece prasina is interesting because the envelope in this species 
is easily penetrated by stains, and so intravital methods of staining were 
possible. The action of methylene blue on the living cell is as follows : 
the metachromatin granules in the central region take up the stain almost 
immediately, and in a short time have stained so intensely that the central 
region is marked off as a dark mass ; one or two granules in the periphery 
also stain. In the peripheral region of the cell the reticulum can clearly be 
seen stained a little darker than the ground substance, and also the unstained 
cyanophycin granules (Fig. 14). 
