186 Prof. H. Karsten on the Formation, 
of the fission-theory account not only for the nested membranes 
occurring in every individual cell, but even for the general cel- 
lular envelope of the entire organism (cuticle), and also for the 
intercellular substance, at least as far as the existence of the 
latter is admitted by them. 
According to this theory, the outer thickening layer of the 
primitive, freely produced cell, which forms the basis of the deve- 
loping organism, must be the commencement of the enveloping 
membrane ; it is produced whilst the cell, constantly increasing 
in volume, “has its space repeatedly divided into smaller com- 
partments by fold-formation of its inner layer (the primordial 
sac). 
Each of the cells thus produced is supposed to secrete the 
connective mass (intercellular substance) which unites them 
into a coherent tissue, just as the various layers of which the 
cell-wall consists are secreted externally and internally by the 
primordial sac. 
On the other hand, those histologists who believe that cells 
do not originate by constriction, but as independent structures 
within the fluid contents of the mother cell, and who are con- 
vinced that, along with the production of laminz by the assimi- 
lative faculty of the cell-wall, there is also a simultaneous che- 
mical change, and in many cases a remarkable regeneration of 
the mother cell by the endogenous development of daughter 
cells—such observers dissent from the previous views regarding 
the origin of intercellular substance only so far as to assume 
that the growth of laminz does not arise from an excretion of 
the original cell-membrane (the primordial sac), but by intus- 
susception into its mass. They also conceive that the inter- 
cellular substance, which is doubtless present in the interspaces 
of the active cells, was at one time the outermost cell-membrane 
or layer of a cell-membrane, but that this has become changed 
by the agency of assimilation in such a manner that it is sub- 
jected to the solvent power of the nutritive fluid which soaks 
the vegetable tissue and becomes received into its mass. 
The explanation of the origin of the membranous envelope 
(cuticle) as an excretion of the epidermis does not harmonize 
with the visible peculiarities of this lamina as pointed out. by 
Brongniart, who describes it as a delicate homogeneous covering 
of the epidermis; for should the laminz of the cell-wall, toge- 
ther with the cuticle, arise simply by excretion from the cells, 
the homogeneous nature of this membranous investment would 
be destroyed by the first act of division of the germ-cell, as it 
would then be secreted first by two and soon afterwards by four 
or many cells, and finally by the epidermic layer. In accordance 
with this mode of origin, it would rather have presented a struc- 
