Pritsch, Studies on Oyanophvceae. 
211 
It sliould be notecl that (as exemplified by Aphanothece and 
Gloeothece) a cliange in tlie character of tlie external mucilage 
was a necessity for tlie formation of tlie filament. In OscUlafia 
tlie external Investment lias been discarded altogether and tlie 
necessary rigidity is obtained by tlie colierence of tlie cell-sheatli: 
in other fornis liowever (Ly)igbya) tliis was not snfficient and 
a more or less consistent out er slieat li was further developed. 
Tlie heterocystous fornis studied liave all retained an out er 
slieatli and perbaps it is owing to tliis that coherence of tlie 
cell-sheatli is not so marked in any of these fornis 1 ). The 
scheine 011 p. 46 is meant to show the way, in which the higher 
filamentous Cyanophyceae arose froni tlie unicellular fornis, as 
far as can be gathered froni present day forms: but although 
it indicates relationships it must not be regarded as a phylo- 
genetic series. As will be seen two main series of forms are to 
be distinguished: the series Gloeocapsa — Gloeothece — Ana- 
baena , in which tlie cell-sheatli can always be recognised as 
individual to each cell, owing to its very marked constriction, 
even ivlien coherent; and the series Oscillaria — Lyngbya , which 
probably developed froni a form like Synechococcus , and is 
characterised by the uniform cell-sheatli around the whole fila- 
ment. Lyngbya represents a return to the old conditions, in as 
much as it possesses a well-marked external slieatli, which is 
lacking in Oscillaria. 
Summary. 
It may be well to brielly summarise tlie conclusions of the 
present paper: 
(I) Each cell of the sporogenous filament (and probably also 
of mature vegetative filaments) of an Anabaena has two enve- 
lopes, — an inner investment, whicli completely encloses the 
protoplast, and outside tliis a special cylindrical slieatli, which 
has been designated tlie cell-sheatli; when division of the cells 
takes place tliis cell-sheatli is simply split into two fresh sheatlis 
by the development, of an intercellular septum. 
(II) The inner investment, which is possibly the only one 
in young stages, is regarded as a modified plasmic membrane 
of a viscous, gelatinous nature: the cell-sheatli is probably a 
modified innermost layer of the external mucilaginous slieatli 
and unlike tlie inner investment is dissolved by chromic acid, 
except in the almost mature spore. 
b As already liientionecl tlie liiglier lieterocystoiis fornis liave not been 
fully examined, but it may be well to point out tliat tlie heterocystous 
forms in Kirchner's Scytonemataceae (98, p. 78, fig*. 57 C and D for in- 
stance) appear to have the sanie moniliform structure of the cell-sheath of 
the filaments inside the external sheath as Tolypothrix. Plectonema (loc. cit. 
fig. 57 A), which is devoid of heterocysts on the other hand appears to 
exliibit a structure like that of Lyngbya. All these fornis will be treated 
of subsequently in greater detail. 
