132 THE PHENOMENON OF 



in families, either loosely (by thin or gelatinous enveloping 

 membranes), or more firmly (by tougher cell-coats). The 

 second case, in which the reproductive cells show impor- 

 tant differences from the vegetative generations, is repre- 

 sented by a few Palmellacese with swarming transitional 

 generations, and by the Diatomacese. In the former the 

 reproductive cells originate, like their predecessors, by 

 division, but they soon assume a peculiar form and, fur- 

 nished with cilia, break through the enveloping membrane 

 of the old family to commence the vegetative cycle anew, 

 after a short period of motion ;* in the latter, the 

 Diatomaceas, the transitional generation is formed in a 

 way essentially different from the preceding division - 

 generations, for two cells become connected by simple 

 or double conjugation, and, form one or two reproductive 

 cells by the combination of their contents at the point of 

 connection ; these reproductive cells are originally globular 

 and altogether unlike the very peculiarly shaped vegeta- 

 tive cells of this family, but, by an uninterrupted growth, 

 soon acquire the shape of the parent-cells, from which 

 the first generation of the new cycle originating in this 

 way is distinguished merely by greater size.f 



* Vide the geiiera Tetraspora, (Nag., t. ii, c) ; Didyospharium, (Nag., 

 t. ii, E) ; Apiocystis, (Nag., t. ii, A) ; the first of which forms a cellular 

 plate, the second a free solid group of cells, the third a solid group attached 

 by an attenuated base like a stalk. Besides the alternation in the direction 

 of the divisions, there occurs in these genera a further slight distinction in 

 the generation of cells, in that one or two generations of cells are alternately 

 transitory, that is, they divide anew before they have attained the usual size 

 of the vegetative cells. 



f Vide Thwaite's 'Observations on the Diatomacese,' 'Ann. and Mag. of 

 Nat. Hist.,' vol. xx, (1847,) t. iv; (Eunotia turgidd), t. xxii; (Cocconema 

 lanceolatum, Gomphonema minutissimum Himantidium pectinale), series 2, 

 vol. i, (1848,) t. xi, (Melosira and CycloteUa.} The denomination of the 

 reproduction cells produced through conjugation as sporangia (sporangial 

 frustules) by Thwaites, depends on a too widely stretched comparison with 

 those of the Desmidiacese. According to Thwaites's own description, the 

 reproductive cells of the Diatomacese pass directly into vegetative cells, 

 which is not the case in the Uesmidiacese. The strange phenomenon that 

 the primary generation formed through the conjugation attains about double 

 the size of the parent cells, is simply explained by a gradual decrease of size 

 in the series of vegetative generations formed by division, a phenomenon to 

 which I have already called attention above in the case of Glaocapsa. 



