428 Griffiths.—On Glaucocystis Nostochinearum , Itzigsohn. 
resembling true karyokinetic division. Darkly staining granules in the 
cytoplasm have been recorded under the names of central granules, meta¬ 
chromatin, and volutin, and their gradual disappearance by diffusion has 
been noted (Guilliermond (’ 12 ), Minchin (’ 12 )). 
Miss Acton (’ 14 ) has described the nucleus of Chroococcus. In C. macro¬ 
coccus it is confined to a definite area, and is more or less oval in form. It 
contains chromatin, but has no nuclear membrane or nucleolus. It divides 
by simple transverse fission. In other species the nucleus is of the ‘ open ’ 
type. In Merismopedia elegans no nucleus is present in certain stages, but 
later on granules of metachromatin appear in the cytoplasm, followed by the 
formation of a deeper staining nuclear area. The latter develops darkly 
staining granules, while the metachromatin disappears from the cytoplasm. 
The nucleus divides, apparently by a transverse fission, and subsequently 
degrades and disappears. The cytoplasm commences to constrict before 
actual nuclear division, and the final division is completed after the division 
of the nucleus. 
The cytology of Glaucocystis shows a more highly elaborated nuclear 
structure than any other member of the Cyanophyceae, but nevertheless it 
is only a difference in degree and not in kind. In stage (a) described above, 
the nucleus is apparently of the ‘ open ’ type. One might say that the 
karyoplasmic area is only potentially a nucleus. It is continuous with the 
rest of the. cell protoplasm and devoid of stainable material. It is only 
distinguished from the general protoplasm by the marked absence of the 
comparatively large granules of metachromatin. The area is not perfectly 
definite, and is not bounded by a membrane. It is not in any way a vacuole. 
In this stage it is comparable with the ‘ open ’ nuclei of many of the 
Hormogoniae, except that it is of very regular form. 
In stage ( b ) the karyoplasmic area moves to the centre of the cell and 
diminishes in size. The metachromatin granules of the cytoplasm diminish 
in number. At the same time the nuclear area, although without signs of 
a stainable reticulum, is diffusely stainable as a whole. It seems that the 
metachromatic substance is being taken into the nuclear areas in a diffuse 
form. Similar diffusion of metachromatin has been noted in other cases by 
Acton (T 4 ), by Wager and Peniston (’10), and by Lutman (’ll). 
In stage (c) the nucleus has contracted to its smallest size. A karyo¬ 
plasmic reticulum of thicker threads has developed. This stains deeply, 
probably owing to minute granules of chromatin in the threads. At the 
intersection of the threads larger chromatin grains are seen. With haema- 
toxylin these grains take a dull dark red colour. They seem to be solid. 
The metachromatin grains of the cytoplasm are coloured a bright purple-red, 
and appear to be translucent. By this time all the metachromatin has 
disappeared from the cytoplasm. The cytoplasmic reticulum consists of 
granular threads. At this stage, therefore, the cell structure resembles that 
