F r i t s c h , Studies on Cy anophyceae. 199 



process in tlie foUowing words: ..Tlie divi^^ion of tlie cell is 

 brought about by the formation of a transverse wall, whicli 

 grows inwards from the lateral wall and divides the cytoplasms 

 and nucleiis into two equal or nearly equal parts". I was not 

 able to observe the exaet point of origin of the separating mass, 

 but it is very probable that it is formed from without inwards 

 as in the specimens, stndied by Wag er. In accordance with 

 the views, which I have expressed above on the nature of the 

 cellular envelope of Cyano'phyceao, I consider that this intercel- 

 lular substance. which arises between the new daughter-cells, is 

 only a modihed portion of the protoplasm. 



^ Internal to the cell-sheath sporogenous filaments of Anahaena 

 however exhibit a further Investment in the form of a narrow 

 colourless strip of similar appearanee to the intercellular sul)- 

 stance and which like it takes on a faint brown colonration with 

 lodine (fig. 5, 8 i. i.). This apparently abuts directly on the 

 coloured peripheral portion of the cell-contents and is continuous 

 with the intercellular mass, separating adjacent cells. That is 

 to say each protoplast of a sporogenous filament of Änahaeiia 

 is surrounded on all sides by a thin strip of colourless substance 

 (fig. 5 and 8, i. i.), which I shall refer to below as the inner 

 investment; laterally this envelope is bounded by the cell-sheath, 

 already described, whilst 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 dming the commencement of the vegetative phase and 

 to which aU the remarks on the natm^e of the Investment, made 

 above (p. 32) apply; as filaments pass over to the sporogenous 

 condition the cell-slieath begins to develop on the outside of 

 the inner investment. 



If filaments of the Anabaciia in question are treated with 

 ^ 33 7o Solution of chromic acid according to Gomont'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 surroimded 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 already stated. involved in this 

 process. As the spores reach matmity however this treatment 

 has no effect on the cell-sheath and leaves the two Investments 

 of the protoplast well-defined. This seems to point to the fact, 

 that the cell-sheath is a specialised inner portion of tlie mucila- 

 ginous envelope, for according to Gomont the 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, which are in a purely vegetative 

 condition. It is not visible in these cases however without the 

 help of stains, whereas sporogenous filaments admit of the 

 recoornition of all the above structural features in the ordinary 



14* 



