446 Acton. — Observations on the Cytology of the Chroococcaceae. 
an affinity for chromatin stains. These two parts of the protoplast will be 
distinguished as ‘nucleus’ and cytoplasm. 
Sections cut from carefully prepared material showed that the ‘ nucleus ’ 
is reticulate in character. With Delafield’s haematoxylin the whole ‘ nucleus ’ 
appears to stain deeply, but at the nodal points of the reticulum granules 
are present which stain more intensely than the body of the ‘ nucleus ’ 
(Figs. 16 and 17). With iodine-green-fuchsin the body of the ‘nucleus’ 
does not appear to stain at all, or else takes on a slight bluish tinge, but the 
granules stain an intense blue and so the ‘ nucleus ’ is sharply defined. The 
reticulum stains red, as does the cytoplasm. 
In cutting the material, sections thinner than ioju were found to be 
unsatisfactory. Unfortunately, this means that the section does not 
necessarily pass through the ‘nucleus’, and, in fact, only two sections 
through the ‘nucleus’ were found. These are shown in Figs. 16 and 17, 
and prove beyond doubt that the portion of the ‘ nucleus ’ which reacts with 
chromatin stains is confined to the surface. Traces of a network could be 
seen inside the ‘ nucleus ’, but this may have been the inside view of the 
surface reticulum. 
The author is inclined to think that the interior of the ‘ nucleus ’ consists 
of a sap vacuole, but a thin slice through the ‘ nucleus ’ which would show 
this clearly has not been obtained. This suggestion is supported by the 
fact that the ‘nucleus’ frequently appears flattened and in a state of 
collapse. 
At first sight the cytoplasm appears to be coarsely reticulate, except in 
the neighbourhood of the ‘ nucleus ’, but closer investigation shows that 
a fine reticulum is present which passes insensibly into the reticulum of the 
nucleus. This certainly corresponds to the reticulum present throughout 
the ground substance of the other members of the Chroococcaceae, but its 
definite character is disappearing. 
The coarsely reticulate appearance is due to the presence of cell-sap 
vacuoles, which break up the ground substance. The amount of vacuoliza- 
tion varies, being much more marked in the older cells ; the region 
immediately surrounding the ‘nucleus’ is the last to be broken up. The 
effect of this vacuolization is to destroy the regularity of the finer reticulum, 
which becomes very much distorted, and in parts can hardly be distinguished 
(Fig. 1 5). The thickenings at the nodal points, which are characteristic of 
the network in other types, have almost disappeared, but traces of them still 
remain, especially near the ‘nucleus’ (Fig. 15)* Metachromatin granules 
are scattered through the cytoplasm. 
In Chroococcus macrococcus there is evidently a distinct morphological 
differentiation in the protoplast. The fine reticulum, characteristic of the 
whole protoplast in simpler types, has been restricted (in its definite 
character) to one portion of the cell, and associated with it is a very definite 
