244 THE PHENOMENON OF 



cell. The imperceptible increase of length during the 

 rapid course of this process, is a proof that the alteration 

 of position of the nucleus is a real change of place, and 

 not effected by a one-sided complementary growth of the 

 cell (like that in the Desmidiacese). Hence we ordinarily 

 find the nucleus again in its characteristic central position, 

 as in the older, larger cells, even in newly-formed cells 

 scarcely more than half the length of the mother-cells. 

 When the cells have attained the normal dimensions, by 

 doubling their length, the process of division is repeated, 

 till at length fructification commences. Excepting the 

 processes relating to the nucleus, no perceptible pheno- 

 mena of solution occur as preparatory to the division of 

 the cells ; in particular, no change is detected in the 

 green bands or the starch-grains existing in them. I 

 have a little more to add in regard to the cell-membrane, 

 especially the relation of its different layers to the series of 

 generations of cells. In the larger Sipirogyra a double 

 membrane may be clearly detected : an outer, which 

 envelopes all the cells of the filament in common, which 

 I will call the cuticle (Ueberhaut, cuticula) ; and an inner, 

 the true cell-membrane, which, when superficially ex- 

 amined, seems to belong to the individual cells, but, in 

 reality, represents a very complicated system of encase- 

 ments. Between the outer and inner coat, we may 

 sometimes distinguish by its darker colour a membranous 

 layer, which however is neither a simple nor a special 

 membrane, but belongs to the system of layers of the 

 internal membrane. The cuticle is very thin in Sp. nitida 

 and juyalis, at most ^ millim. thick, more transparent 

 than the proper cell-coat, from which it is distinguished 

 also by its behaviour with acids and solution of potash, 

 as it does not swell up. On the surface it exhibits very 

 numerous, irregularly scattered, sharply circumscribed, 

 circular bright points, scarcely ^th millim. in diameter, 

 which in the sectional view of the cuticle appear as 

 streaks running crossways through it. These streaks 

 are still more distinctly seen in Sp. lubrica, in which the 



