IIEJUVENKSCENCE IN NATURE. 245 



cuticle (j^) is far thicker than the cell-membrane. When 

 tincture of iodine is applied, the said streaks acquire a 

 brown colour, while the mass of the cuticle remains 

 uncoloured, which, as well as the general aspect, makes 

 them look like little canals perforating perpendicularly 

 through the cuticle. We have no observations on the 

 mode of origin of this cuticle, but I should rather incline 

 to regard it as an incrustation- membrane, than as the 

 primary cell-membrane of the first cell of the filament. 

 The more complicated structure of the proper cell-mem- 

 brane only becomes evident after long action of solution 

 of potass, when the membrane, about ^ijth millim. thick, 

 swells up to almost three times the thickness, and dis- 

 plays clearly at least the principal constituents of which 

 it is composed. It consists, as examined from within 

 outward, of a series of layers, the inmost of which clothes 

 one cell only, the" second two, the third four, the fourth 

 eight cells, &c. The first and second layers, from within, 

 are most clearly distinguishable, and one or other of them 

 is the thickest of all, according to the age of the cells ; 

 the further outwards, the thinner and less distinguish- 

 able become the lamellae, so that usually only four of 

 these can be distinguished, the outermost of which, how- 

 ever, (of course only in old filaments composed of 

 numerous cells,) we must regard as composed of several. 

 These various layers are the membranes of so many 

 different cells encased one within another. The inmost 

 layer alone constitutes the special cell-membrane of the 

 individual actually vegetating-cell ; at the time of the 

 full development of the cell, it is thickest of all. The 

 second layer is the membrane of the immediately pre- 

 ceding mother-cell, it therefore encloses two cells, and 

 becomes attenuated by the extension which it must 

 necessarily undergo during the pei'iod of growth of the 

 daughter-cells, so that it is only about half as thick as 

 the fully-developed membrane of the daughter-cell. The 

 third layer is the cell-membrane of the penultimate 

 mother-cell (the grandmother-cell) ; it encloses four cells. 



