90 
W. B. Crow 
met with in many species of Aphanocapsa and Microcystis where the 
cells are embedded in copious mucilage. No doubt the optical 
properties of the latter are partly responsible for this. But it must 
also be remembered that the quality of the mucilage-vestment sur¬ 
rounding a cell would play a very considerable part in determining 
the surface tension and consequently the formation of a membrane. 
Where present the membrane is not separable from the protoplasm 
by plasmolysis, neither do stains differentiate it clearly. In all the 
species which we have examined, where there is any trace of a mem¬ 
brane at all, the new part of this layer does not grow into the proto¬ 
plasm as a ring-like septum, finally closing and splitting into two as 
in the filamentous forms, but instead there is a constriction of the 
cell into two halves, the membrane growing round each daughter 
cell as it is completed. All such facts indicate that we are dealing 
with a hyaloplasm or modified outer layer of protoplasm rather than 
with a cell-wall. 
The Mucilage 
The real cell-envelope in the Chroococcaceae is the mucilage 
sheath. Mucilage is a common form of secretion in the Thallophyta 
and even in the higher plants, but its formation is generally localised 
and it helps to build up definite morphological features. The uni¬ 
cellular Cyanophyceae, however, show very primitive features in 
their mucilage formation. Nearly all forms produce it at the peri¬ 
phery of the cell whilst in some, such as Chroococcus turgidus (Kuetz.) 
Naeg. it accumulates within the cytoplasm 1 (5). Of course it is in the 
form of superficial deposits that it reaches its greatest development, 
and as such it soon takes on a protective function. In 1901 
Lemaire(i3) showed in species of Chroococcus and Glceocapsa, in some 
of the Nostocaceae and in Glceotrichia, that the mucilage sheath showed 
great analogy with the pectic substances of higher plants. It readily 
took up basic anilin dyes, but had no affinity for acid dyes. In con¬ 
trast it was found that in some Stigonemaceae and less commonly in 
the Oscillatoriaceae the sheath consisted mainly of a complex product 
which seemed to be due to the combination of an acid compound, 
filling the same role as pectic acid, with another substance of basic 
composition. The latter, which Lemaire calls schizophycose, is in¬ 
soluble in zinc chloride and in dilute acids and alkalis. It is coloured 
by anilin blue and certain acid anilin dyes, but has no affinity for 
basic colours like ruthenium red, neutral red, etc. Finally, in some 
1 The so-called mucus vacuoles of many forms do not show reactions for 
pectins and so must be distinguished from the above. 
