Development, and Structure of the Vegetable Cell. 288 
after the thickening of their walls and of those of their mother 
cell has commenced, coincidently with the more distinct protru- 
sion of the dissepiment, that the application of solution of ehlo- 
ride of zine and iodine produces a blue coloration of the external 
membranes of the new joint-cells and of their now likewise 
thickened and closely embracing mother cell; and this happens 
contemporaneously with the detachment of a secondary cell, 
which up to this period could not be isolated. 
The difficulty, or perhaps, at present, more correctly speaking, 
the impossibility, of effecting, by the agency of endosmosis, the 
separation of the closely approximated membranes of the two 
endogenous cells from their mother cell (the secondary membrane 
of the joint-cell), or, before the thickening of their walls, from 
the secondary cells within them, is owing in part to the nature 
of the intercellular substance, and in part to their very similar 
chemical and physical (diosmotic) properties. 
The similarity or identity of these membranes is so great, in- 
deed, that it is extremely difficult to determine whether the young 
cells consist of a single or double cell-wall, and whether the en- 
veloping secondary membrane of the mother cell is still present 
or has been destroyed. It is moreover somewhat difficult to 
make out with certainty the presence of the large vesicles filled 
with colourless fluid amid the corpuscles of chlorophyll and 
starch within the cavity of the cell. . 
In many species of the genus Spirogyra, and still more easily 
in Cladophora glomerata, we may ascertain, by cutting through 
their joimt-cells under water, or by the action of different re- 
agents, that these cells are filled with a delicately walled cellular 
tissue. And we may sometimes be so fortunate as to witness 
the complete extrusion of the new joint-cells (themselves also 
occupied by cellular contents) from the mother cell before their 
membranes are thickened. 
In Cidogonium, the joint-cells of which acquire cellular con- 
tents only after they have attained their full size, and simulta- 
neously with the commencement of the thickening of their walls, 
these are no longer protruded from their mother cell ; and in their 
younger phases (in which, as they then only exhibit fluid con- 
tents, they cannot be distinguished from the large neighbouring 
secretion-cells) they are furnished with walls as delicate as those 
of the latter, and are immediately dissolved by contact with 
water. Indeed it is only when sections of such plants are made 
in a weak solution of gum-arabic, instead of water, that we can suc- 
ceed in observing for any length of time the very delicately walled, 
easily overlooked, endogenous, non-nucleated cells emerging 
from their parent cell; and even then it is impossible, for rea- 
sons already stated, to distinguish whether these are only trans- 
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