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 in 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, I 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 secondaiy joint-cells occurs also in those Spirogyra 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- 



