426 Prof, H. Karsten on the Formation, 
In Cladophora glomerata, whose endogenous cells have just 
been considered, a thickening of the originally very delicate 
membrane of certain endogenous cells occurs regularly, but only 
when these have acquired their normal size and by their contact 
and pressure concur in the formation of a transverse septum 
whilst, at the same time, the secretion-cells in their vicmity un- 
dergo absorption. 
The more rarely occurring conditions of development described 
in the last section seem to me also to elucidate sufficiently the 
normal process, the essential relations of which can with diffi- 
culty be followed in the joint-cells when completely filled with 
chlorophyll and other secretion-materials. 
Fig. 46 shows one of the joint-cells of Cladophora poor in 
chlorophyll and filled with the large colourless cells, such as are — 
produced by cultivation in pure distilled water, especially when — 
the plant has been cut into short pieces. The specimen has — 
been subjected for a short time to the action of dilute glycerine, — 
in order to detach the endogenous cells from the membrane of — 
the mother cell. The membranes of these cells, though sepa- — 
rated by a delicate septum, were not perceptibly thickened; by — 
the action of exosmose they were contracted and also detached — 
from each other at the septum (z), without leaving a layer of © 
deposit between them. ‘This happens also im the case of Cido- — 
gonium, as above described (p. 283), but appears to be of un- 
usual occurrence. 
The conviction is thus arrived at of the actual presence of — 
free cells and of the origin of a septum by the mutual contact 
of their walls, even before any appreciable thickening, and with- — 
out the agency of any involution or folding of the mother cell. — 
Nageli, indeed, noticed these phenomena, but nevertheless be- 
lieved in the existence of inward-growing walls, which might be — 
invisible either on account of their tenuity or of their containing | 
a great amount of water. F 
One of the two thin-walled daughter cells represented in — 
fig. 46 is seen at g to be again divided by a distinguishable — 
septum, in all respects similar to the septum displayed in Gido- — 
gonium, as observed at its origin and represented in figs. 21-29, — 
and described at p. 277. This delicate septum would not be — 
distinguishable if the inner wall of the mother cell were, as 
usual, coated with secretion-vesicles, until these were absorbed, — 
and the later process of thickening set up in it. In the present — 
case, the observation of this young joint-cell, which had been ; 
rapidly expanded in consequence of the injury done to the 
neighbouring cells, was rendered possible by the less degre of 
aggregation of the secretion-materials. ) 
The cells that make their appearance on the section of a joint- 
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