Acton . — Observations on the Cytology of the Chroococcaceae. 443 
Gloeocapsa sp. is worthy of special mention, because Olive (’ 04 ) has 
definitely described mitosis in Gloeocapsa. He says : ‘ The segmented 
spireme in Gloeocapsa appears to consist of a simple more or less spiral 
thread, having about eight chromatin granules held by the linin and situated 
in the middle of the cell, with its long axis corresponding to the long axis 
of the cell ’ ; and again, ' Finally, the most necessary requirement of mitosis 
is fulfilled in that a longitudinal fission of the chromosomes occurs. This is 
plainly evident in the case of Gloeocapsa , in which the simple spireme threads 
divide lengthwise, beginning at the two ends and splitting thence pro- 
gressively to the middle of the thread.’ 
Now in Gloeocapsa sp. the reticulum connecting the plasmatic micro- 
somes in the centre of the cell appears to stain more deeply than the 
remainder of the reticulum (Fig. 13). This certainly simulates a segmented 
spireme stage, and is of quite frequent occurrence in some preparations. It 
probably accounts for Olive’s ‘ mitosis ’. It may be a stage in the specializa- 
tion of the central portion of the network, but is possibly an artifact due to 
slight shrinkage of the central region. 
The genus Merismopedia differs rather strikingly from the species 
described up to the present, since one species, at any rate, has an ‘incipient 
nucleus’ 1 in the dividing stages. Two species only were available — 
M. elegans and M. glanca. The former occurred very sparingly in some 
plankton material from Sutton Park, which also contained Chr. limneticus 
in fairly large quantities. M. elegans was unfortunately overlooked when 
the slides were prepared, and this species was in consequence overstained in 
most preparations. One slide, however, stained with Delafield’s haemato- 
xylin showed a fairly large colony with cells in all stages of division. 
Some of these are shown in Figs. 8 and 9. The stain had been completely 
washed out, with the exception of a deeply staining portion which is 
evidently the ‘ nucleus \ The position of this in the dividing cells is shown 
in Fig. 8. The cells also contained a few colourless refractive granules, 
which were probably metachromatin granules. 
The ‘nucleus’, before division takes place, is situated in the centre of 
the cell. Constriction of the cell takes place in two planes simultaneously, 
but the ‘nucleus’ appears to constrict, if not to actually divide, before it is 
reached by the constriction of the cell-wall (Fig. 9, b). 
The structure of the ‘ nucleus ’ is somewhat similar to that of Chr. 
macrococcus , which will be described later. It is a restriction of deeply 
staining plasmatic microsomes to a small definite area in the network. 
There is some indication that this substance is distributed along the network 
1 The term ‘ incipient nucleus ’ has been suggested by Professor G. S. West, in his forthcoming 
publication in the Cambridge University Press, 1914, to describe the nuclear structure found in the 
Myxophyceae, and the word ‘ nucleus ’ will be used in this sense in the remainder of the paper. 
The reasons for using the term are given in detail by Professor West, and it is therefore unnecessary 
to discuss them here. 
PI h 
