450 A clou . — Observations on the Cytology of the Chroococcaceae. 
of chromatin becomes connected with the part division and is confined, 
in highly developed cells, to that part of the protoplast which is connected 
with division, i. e. the nucleus. 
If this suggestion as to the lines on which nuclear structures have 
evolved be true, then a stage should exist in which part of the reticulum is 
definitely marked off and true chromatin should be present, also this por- 
tion of the reticulum should have a definite relation to division of the cell. 
This stage is actually found in Chroococcus macrococcus (Fig. 15). 
In this species a small portion of the reticulum always differs very markedly 
from the rest of the protoplast. The plasmatic microsomes contain true 
chromatin, and this portion of the reticulum has the power of dividing 
before the cell-wall constricts. 
The peripheral portion has also begun to change in structure. The 
fine reticulum can still be seen in places, especially in the neighbourhood of 
the nucleus, but the plasmatic microsomes have almost disappeared, and the 
reticulum is very much broken up and distorted by cell-sap vacuoles. 
The nucleus in Chr. macrococcus seems to remain permanently in a 
condition which resembles the resting nucleus of higher plants, except that 
a nucleolus and nuclear membrane are not present. 
The exceedingly small and delicate reticulum found in these unicellular 
forms has not been described for filamentous species. Probably it does not 
occur, though this needs verification. 
Vacuolization may have entirely crushed out this fine reticulum in the 
cytoplasm, giving rise to a coarser one, and at the same time the reticulum 
of the nucleus may have become irregular and thickened, giving rise to 
the achromatic reticulum of Guilliermond (’ 05 ) with chromatin granules 
embedded in it, or to something approaching the highest modification of 
Gardner’s diffuse type of nucleus, in which ‘differentiation seems to be 
proceeding towards the spireme formation found in the dividing nucleus 
of the higher plants It is not unlikely that the ‘ spireme ’ condition 
of the nucleus is a permanent one in some low types. This is, however, 
merely a suggestion, and the filamentous forms have yet to be investigated 
from this point of view. 
VIII. Conclusions. 
1. In the Chroococcaceae a highly specialized nucleus of the type found 
in higher plants does not occur. 
2. There is a gradual transition in the structure of the cell from an 
almost undifferentiated condition in the lower types to a somewhat 
specialized one, of which Chroococcus macrococcus represents the highest type 
examined, and Merismopedia etegans an intermediate stage. 
3. The protoplast consists of a ground substance traversed by a reti- 
culum of delicate threads, with thickenings at the nodal points. These are 
