REJUVENESCENCE IN NATURE. 147 



which is formed by a vegetative permanent-cell, like the 

 boundary cell of Nostoc. In Tolpothriw and 8cMzosiphon 

 several such vegetative permanent-cells frequently occur 

 at the base of the filament, and in these very genera, as in 

 Scytonema and Euactis, the formation of them is repeated 

 at intervals in the course of the filament, vi^here they 

 appear as interstitial cellSj which by subsequent parting 

 of the filament, and the lower part overgrowing the 

 upper, become basilar-cells of seeming branches. In the 

 Scytonemese, the filaments are of pretty uniform structure 

 throughout the whole length, but in the Rivulariese they 

 run out above into a more slender capillary point or 

 flagellum. It is not yet made out whether the distinction 

 of the upper and lower end of the filament of the Algae 

 of this group is caused by an apical growth continued on 

 beyond the formation of the first basilar cells, but this 

 nmch is certain, that the cells of the filaments multiply, 

 in their further growth, as in the Nostochinese, by repeated 

 halving, which, however, does not terminate simultane- 

 ously in all parts of the filament, but continues longer in 

 some parts than in others. In the Scytonemese it is 

 carried on farther in the points (not merely of the entire 

 filaments, but also in the individual sections).* In the 

 Rivulariaceous genera Mastichothrix, Masticlionema, Bivu- 

 laria, &c., in the lower ; in Euactis, in the intermediate 

 parts. With this is associated a distinction of the cells 

 belonging to the different parts of the filament, becoming 

 continually more evident with the progress of the division, 

 and especially marked in the last generation. In the 

 Scytonemese the cells pass into germ-cells, in the last 

 generation, mostly only at the upper part of the filament ; 

 in Euactis only in the intermediate ; in Mastichothrix and 

 Masticlionema only in the lower, and thicker, not in the 

 upper capillary portions ; in Rivularia, lastly, the cells of 

 the lower thicker part exhibit another difference, for the 



* In many species of Scytonema and Tolpethrix the lower end of the 

 sections may grow out in this way into new points. 



