130 Prof. H. Karsten on the Formation, 
cedes the thickening of the cell-membrane. Before this transi-’ 
tion-state the cell-membrane is more delicate, but more elastic ; 
it then loses its elasticity, becomes thicker, appears to be swelled 
up and nearly gelatinous, and finally becomes again condensed 
and solid. 
When the absorption of the chlorophyll-sac is completed above 
the new septum, it then probably advances towards the ends of 
the mother cell (the alteration of the membranes of the neighbour- 
ing cell-membranes and the production of new chlorophyll in the 
interior of the daughter cells going on simultaneously), and the 
contraction of the unthickened membrane of the daughter cell, 
in consequence of the action of dilute diosmotic fluids, exhibits 
the customary appearance, the separation of the membranes of 
the septum taking place at length, not in the centre, but in the 
periphery (figs. 76&77). It then presents a great similarity to 
those in which the mother cell is still undivided (figs. 78, 79). 
But in the former case the contracting membrane of the mother 
cell tears in the middle between the two contracting daughter 
cells; in the latter, if a rupture takes place, it is im the vicinity 
of the ends of the mother cell. 
In order to explain this mode of formation of the septum by 
fold-formation, we should have to assume here that the fold of 
the membranes of the joint-cell grows through the cavity of the 
cell from one side to the other, commencing always from the 
side opposite to the chlorophyll-sac, and terminating at the 
opposite wall by applying itself closely thereto (figs. 74, 75). 
In opposition to this supposition, 1 may state that I have 
never yet seen an ingrowing fold of this kind in the long and 
thin-jointed species which I have observed, but that I have very 
frequently watched the formation of the septum in all its stages, 
from the first moment at which it is recognizable as a delicate 
and scarcely measurable membrane stretched transversely across 
the cavity of the cell, with the perfectly continuous chlorophyll- 
sac passing close beside it as above described, up to the com- 
pletion of the absorption of the latter at the boundary of the 
septum, which has in the meantime been increasing in thickness. 
This mode of septum-formation by means of daughter cells of 
the secondary joint-cells occurs also in those Spirogyre which 
contain several chlorophyll-sacs in their joint-cells, and perhaps 
quite as frequently as the one above described (p. 124) by the 
daughter cells produced in the nuclear cell. 
In both cases the presence of several chlorophyll-sacs enables 
us to determine with perfect certainty whether, simultaneously 
with the production of the septum by endogenous cells, a fold- 
formation of the mother cell has or has not taken place. Even 
when the daughter cells of the secondary joint-cells formed the: 
