Development, and Structure of the Vegetable Cell. 31 
the secondary cell appears indeed to have lost its delicate 
though firm consistence. In such cases the chlorophyll-layer 
is found to be surrounded by a thick, almost gelatinous, but 
viscous layer, capable of being drawn out in threads, which it is 
often difficult to separate by endosmotic agents from the mem- 
brane of the primary cell. In this layer, which hkewise ex- 
hibits the reaction of celluline, the contracting chlorophyll-sacs 
leave behind them the impression of their forms as furrow-like 
depressions (figs. 65 & 66). 
This state of aggregation of the membrane of the secondary 
cell appeared to me to prevail especially among plants whose 
joint-cells were in process of multiplication. 
The Spirogyra dubia (Kutzing), represented in fig. 62, had been 
immersed for some time in carbonic-acid water, when the mem- 
branes of the secondary cells contracted by the action of a watery 
solution of iodine, but appeared altered, almost corroded, and in 
many cells ruptured at the ends during the contraction, by 
which means the endogenous cells (a) were enabled to escape 
from them. These cells contained a great abundance of the 
above-described celluline, which acquires a red colour by treat- 
ment with glycerine and iodine, and, in the course of the further 
growth of the plants m water containing carbonic acid, appears 
to be absorbed. 
In those specimens of Spirogyra which have grown for a long 
time in pure water destitute of nitrogenous compounds, the de- 
licate membrane of the secondary cell is completely contracted 
with much facility by the action of a dilute solution of chloride 
of calcium, and is seen to contain no endogenous cells except 
the chlorophyll-sacs. Moreover no celluline is discoverable, 
although the chlorophyll-sac still usually contains the well- 
known large starch-vesicles. 
The membranes of the several cells are not coloured blue by 
the reagents above mentioned ; and I observed a cellulose re- 
action in them, as well as in those of Cladophora, occasionally 
only, and as the exception, and then without being able to detect 
the circumstances upon which this condition of the cell-mem- 
brane depended. 
It follows distinctly, from what has been stated, that the hy- 
pothesis that the joimt-cells of Spirogyra are filled with a tissue 
of endogenous cells is perfectly well founded; for mere va- 
cuoles in a mucilaginous material would not at one time enlarge 
and at another contract by diosmotic agency, nor would they 
possess special contents, and in these again contain cellular 
structures. 
With respect to the nature of the two sorts of cells existing 
within the joint-cells of Spirogyra, and also with respect to 
