132 THK PHKNOMENON 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 Pulmellacese with swarming transitional 

 generations, and by the Diatomaceae. 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 

 Diatomacese, 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 geuera Tetrtispora, (Nag., t. ii, c) ; Didj/ospluBrium, (Nag., 

 t. ii, e) ; Apiocj/stis, (Nag., t. ii, a) ; tlie first of wliicli forms a cellular 

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

 oy an attenuated base hke a stalk. Besides the alternation in the direction 

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

 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. 



t Vide Thwaite's 'Observations on the Diatomaoefe,' 'Ann. and Mag. of 

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

 lanceolatum, GompJionema minutissinrnm HimanUdinm pectinale), series 2, 

 vol. i, (1848,) t. xi, {Melosira and Ci/clotella.) 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 DesmidiaoeBe. According to Thwaites's own description, the 

 reproductive cells of the Diatomacese pass directly into vegetative cells, 

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

 the primary generation formed through the conjugation attains about double 

 the size of the parent cells, is bimply 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. 



