Fritsch, Studies 011 Cyanophyceae. 
201 
stucliecl, — a point of difference from tlie cases, described by 
Borzi (cf. p. 33). Tlie latter author does not describe the way, 
in which tlie two walls of tlie spore develop in Anabciena. I 
leave a fnrtlier discussion of the spore and its relation to the 
„gonidia“ of Brand (03, p. 44 et. seq.) to tlie next paper of 
this series x ). 
In the examination of well-advanced sporogenons filaments 
numeroiis stages are met with (hg. 7, 8), which seenr to me to 
quite plainly Support tlie tlieory of strncture of the investment. 
proponnded in tlie preceding pages. Specimens, such as that 
represented in hg. 8 are of quite common occurrence: liere the 
uppermost spore is about to liberate its contents and the ter¬ 
minal (transverse) portion of the inner investment is more or 
less papillosely developed on one side and this papilla quite 
visibly protrudes tlirough the open end of the cell-slieath. 
Again in hg. 7, which represents the contents of a sporogenous 
cell in course of protrusion, one end of the cylinclrical cell- 
slieath quite visibly surrounds the equator of the protoplast, 
which is enveloped in a new inner investment. Such cases will 
be further discussed in the third paper of this series. 
I still wish to add a few words on the behaviour of the 
externa! mucilaginous Investment of the Anabaena towards stains. 
Treated witli Yesuvin it turns brown and is seen to consist of 
a number of successive layers. The innermost, and therefore 
most recent, of these closely follows the outline of the cell- 
sheaths of the individual protoplasts' and thus presents a monili- 
form appearance, indicating the excretive activity of each cell. 
These Investments do not include the heterocysts (cf. Brand 
03. p. 44). It is very instructive to watch tlie behaviour of a 
tilament, when Yesuvin is slowly added under the microscope. 
A very wide mucilaginous investment, which was quite invisible 
before, becomes indicated by its margins contracting slightly 
and taking on the brown stain. The contraction goes on very 
slowly but evidently, and at the same time the mucilage acquires 
a darker and darker brown colour: ultimatelv it encloses the 
J 
hlament as quite a narrow sheath, showing one or more layers 
of stratihcation. In all probability many of the layers discer- 
nible during the process of contraction are due to folds. As 
soon as anv cell of a hlament becomes transformed into a 
lieterocyst excretion of mucilage from this cell ceases and the 
stratihcation of tlie mucilaginous envelope seenis in the main to 
b Tlie spore.s of certain Cyanophyceae (e. g. Nostoc microscopicum Carm. 
A. commune Vancli., Grloeocapsa alpina Aäg.) differ in tlie lack of an exo- 
spore, wliicli is replaced by a tliick and consistent mucilage slieatli (cf. 
Brand 03, p. 35, 36). These are all aerial species. These forins are inter- 
esting, as to my thinking they show, that in certain cases the spore does 
not deyelop a special cell-slieath (which is regarded as a modified innemiost 
layer of the external mucilage), but that this strncture is replaced by the 
whole of the outer mucilage becoming more consistent, — a pure case of 
homology. 
