Fritsch, Studies on Cyanophyceae. 
199 
process in tlie following words: „The division of the cell is 
brought about by the formation of a transverse wall, which 
grows inwards from the lateral wall and divides the cytoplasms 
and nucleus into two equal or nearly equal parts". I was not 
able to observe the exact point of origin of tlie separating mass, 
but it is very probable that it is formed from without inwards 
as in the specimens, studied by Wäger. In accordance with 
the views, which I have express® above on the nature of the 
cellular envelope of Cyanophyceae , I consider that this intercel¬ 
lular substance, which arises between the new daughter-cells, is 
only a modified portion of tlie protoplasm. 
Internal to tlie cell-sheath sporogenous filaments of Anabaena 
however exhibit a further Investment in tlie form of a narrow 
colourless strip of similar appearance to the intercellular sub¬ 
stance and which like it takes on a faint brown colouration with 
lodine (hg. 5, 8 i. i.). This apparently abuts directly on the 
coloured peripheral portion of the cell-contents and is continuous 
with the intercellular mass, separatin g adjacent cells. That is 
to say each protoplast of a sporogenous filament of Anabaena 
is surrounded on all sides by a thin strip of colourless substance 
(hg. 5 and 8, i. i.), which I shall refer to below as tlie inner 
Investment; laterally this envelope is bounded by the cell-sheatli. 
already described, wliilst terminally it forms the intercellular 
substance between adjacent cells. This inner Investment I regard 
as the actual membrane of the cell, which is possibly alone 
present during tlie commencement of the vegetative phase and 
to which all tlie remarks on the nature of the Investment, made 
above (p. 32) apply; as filaments pass over to the sporogenous 
condition tlie cell-sheath begins to develop on the outside of 
the inner investment. 
If filaments of tlie Anabaena in question are treated with 
a 33 % solution of chromic acid according to Gromont's method 
the cell-contents are slowly dissolved and the envelope, surroun- 
ding each cell, becomes more distinct. Each cell is then seen 
to be surrounded by a definite membrane, constituted by the 
above-mentioned inner investment, whilst the cell-sheath is now 
by no means so easy to recognise or has disappeared; a certain 
amount of contraction is, as alreadv stated, involved in this 
process. Äs the spores reach maturity however tliis treatment 
has no effect on tlie cell-sheath and leaves tlie two Investments 
of tlie protoplast well-defined. This seems to point to tlie fact, 
that the cell-sheath is a specialised inner portion of the mucila- 
ginous envelope, for according to Gromont tlie sheath is far 
more readily soluble, than the actual cell-membrane (loc. cit. 
p. 214, 215). Its differentiation probably commences at a very 
early stage and its rudiments are probably developed, although 
not sharply marked, in filaments, wliich are in a purely vegetative 
condition. It is not visible in tliese cases however without the 
lielp of stains, whereas sporogenous filaments admit of the 
recognition of all the above structural features in the ordinary 
14 * 
