444 Acton . — Observations on the Cytology of the Chroococcaceae. 
after division, and collects again in the central region when the next division 
is about to occur. 
In this way, diminution in size by repeated division is prevented, and 
the central position of the ‘ nucleus 5 accounted for, since it is obvious that 
the ‘ nucleus ’ as a whole cannot travel to the centre. This suggestion needs 
to be more definitely established, but is supported by the following facts : 
In a group of four cells which were separating after division and increasing 
in size, the ‘nucleus’ was not so deeply stained, while the network was 
fairly distinct, especially round the region of the ‘nucleus’ (Fig. 9, a), but 
in cells showing a deeply stained ‘nucleus’ the network and plasmatic 
microsomes could hardly be distinguished (Fig. 9, b). Again, in one cell 
the network and microsomes were quite distinct, but no ‘ nucleus ’ could be 
distinguished (Fig. 9 ,c). 
Merismopedia glauca , on the other hand, appears to have no definite 
nuclear body. This species occurred in abundance in some collections from 
North Wales, and though most of the colonies showed cells in a state of 
active division, no trace of a nuclear restriction of the network could 
be seen. 
The cells only appear to divide in one plane at a time by a gradual 
constriction of the cell- wall, as in Chroococcus tnrgidus (Fig. 10). Most of 
the cells contained a few very large metachromatin granules, which almost 
filled the cell and at first gave rise to the idea that the cells were reproducing 
by gonidia. 
No indication of a single chromatophore or of numerous small chromato- 
phores was seen in any of the types examined. 
VI. Chroococcus macrococcus, Rabenhorst. 
Chroococcus macrococcus is a member of the Chroococcaceae of frequent 
occurrence in sphagnum bogs. The cells are solitary or associated in 
groups of two or four. They are spherical or somewhat elliptical in shape, 
and have a thick lamellose sheath. The diameter of the cells, including the 
sheath, varied from 30 /x to 64 yut, and of the protoplast from 24 /i to 30^ 
in the specimens examined, but they were mainly from cultures and rather 
less than the usual size. The colour of the protoplast is not constant, but 
varies from a golden brown to a dark brown. At times an orange tinge is 
given to the cells by the presence of several blood-red pigment spots. 
(a) Cell-wall ”. 
The cell-wall is lamellose and often very thick, the outer layers 
frequently splitting away very irregularly (PI. XXXIV, Figs. 20 and 21). 
It appears to consist of alternate layers of two substances. 
In cuprammonia it swells rapidly but is not dissolved in thirty minutes, 
while in concentrated sulphuric acid only the darker layers remain. The 
