Development, and Structure of the Vegetable Cell, 431 
Endosmotic fluids of more potency, saline solutions, acids, 
sugar, alcohol, &c., which detach the several adherent mem- 
branes of the superposed cells from one another, have an exos- 
motic as well as an endosmotic action. 
Hence the membranes of the secondary cells become contracted 
and separated from the primary cells with which they were pre- 
viously in immediate contact, by the action of these reagents ; 
and indeed they detach themselves all round from the external 
membrane, the separation of these membranes of the two cells 
not being perfect until the chemical change has commenced but 
not been completed in one wall. In this case, the membranes 
continue united where the thickening of the contiguous primary 
cell-walls has not yet taken place. In the transverse walls or 
septa this happens, therefore, at the centre, inasmuch as their 
lignification advances from the periphery, or centripetally. The 
young joint-cells consequently continue to adhere closely toge- 
ther, apparently wedged into the central opening of the per- 
forated discoid septum, as is exhibited in the case of Spirogyra, 
in Plate VII. fig. 67. 
This phenomenon has especially contributed to support the 
notion that the adjoining daughter cells constituted originally 
only one single cell, which has become divided by the thickened 
portion of the septum. 
In Cdogonium we may convince ourselves, by direct observa- 
tion, that, notwithstanding these results with reagents, which 
might serve as arguments for the constriction-theory, there is 
present a perfect, though it may be an extremely delicate, septum. 
The delusion is still greater if, in this condition (fig. 67), a 
pressure from one side be exercised on the young septum 
(whether effected by the one-sided operation of endosmotic fluids 
or by a change of tension caused by the cutting of a cell in the 
immediate vicinity of the septum), and chemical reagents be 
then applied to dissolve the cell-membrane which is undergoing 
a change in its chemical constitution. 
That many newly formed cell-membranes, or such as are in 
course of development, are soluble in water, and still more 
readily in acetic acid, I have already shown in my above-men- 
tioned memoir on Conferva fontinalis. The cell-membrane, in 
process of thickening, which occupies the central layer of the 
septum, is dissolved by ammonia and iodine as well as by 
acetic acid. 
Of the above-mentioned solvents of the newly thickened cell- 
membrane, ammonia appears to have the weakest action ; its | 
effect, however, is probably the less striking because it acts 
endosmotically, and not exosmotically, upon the contents of the 
secondary daughter cells. However, after the operation of am- 
