Acton. — Observations on the Cytology of the Chroococcaceae . 451 
‘ plasmatic microsomes ’ and serve as centres for accumulation of reserve 
materials elaborated by the pigmented parts of the protoplast. The nature 
of the accumulation varies in the different regions of the cell. 
4. In the majority of species examined by the author there is no 
definite demarcation of central and peripheral regions, but roughly speaking 
the microsomes in the central region accumulate metachromatin and 
correspond to the ‘ Centralkorner ’ of Kohl, and in the peripheral region 
accumulate cyanophycin and correspond to cyanophycin granules. 
5. Chroococcus turgidas may be taken as an example of this type ; 
though it is possible that it may represent a slightly higher one, since a few 
of the plasmatic microsomes in the central region occasionally give a true 
chromatin reaction. There is also in this species a region, just within the 
cell-wall, in which the plasmatic microsomes are undifferentiated, reacting 
only with cytoplasmic stains. 
6. In Chroococcus turgidus the number of metachromatin granules 
varies greatly in different specimens. If the accumulation of metachromatin 
is excessive, it appears to diffuse into the ground substance, and a period of 
active division sets in. At this time the majority of the microsomes in the 
central region react only with cytoplasmic stains, but one or two very large 
metachromatin granules can usually be seen. 
7. Division takes place in this type by the constriction of the cell into 
two approximately equal halves, caused by the ingrowing cell-wall. Occa- 
sionally the reticulum in the central region seems to stain a little more 
intensely in dividing cells, and there is a tendency for the threads to become 
drawn out in parallel lines, though the cross-connexions are not lost. 
8. Metachromatin may represent a stage in the formation of chromatin. 
9. In Gloeocapsa sp., many of the cells show a deeper staining of the 
network in the central region simulating the * spireme ’ stage described 
by Olive (’ 05 ). This may be a more advanced stage in specialization of the 
central region, but it is probably an artifact. 
10. In Merismopedia elegans , which represents a higher type, there 
is a definite ‘ central body ’ or ‘ nucleus ’ at the time of division. This is not 
of the same type as the nucleus of higher plants, but is simply an accumu- 
lation of chromatin, or some substance allied to it, at the nodal points 
of a small definite area in the centre of the cell. There is some evidence to 
show that this ‘ nucleus ’ gradually distributes itself along the reticulum 
after division, to appear again in the centre of the cell prior to the next 
division. Division of the ‘ nucleus ’ takes place before it is reached by the 
ingrowing cell-wall. 
11. Chroococcus macrococcus represents the highest type found. Here 
there is a definite £ nucleus’ and cytoplasm. Only the peripheral portion of 
the ‘ nucleus ’ stains deeply with chromatin stains, and contains a fine reti- 
culum with chromatin at the nodal points. The interior of the ‘ nucleus’ is 
