REJUVENESCENCE IN NATURE. 291 



or ribands (Hyalotkeca, Didymoprium, Isthmosira), the 

 conjugation does not commence until the filaments have 

 broken up into their individual links. In Bambusina 

 alone, according to Ralfs, occurs the case of isolated cells 

 conjugating with others still constituting links of con- 

 tinuous filaments, and thus becoming attached, like 

 parasites, upon the filaments. The dehiscence of the 

 external cell-membrane is especially evident in Tetme- 

 morus and most Closteria ; it always takes place in a 

 transverse direction, in the middle between two halves. 

 While the dehiscing outer membrane opens more or less 

 widely on one side (like the dehiscing cells of (Edogonium), 

 the delicate internal membrane protrudes as a tubular 

 process, to unite with the corresponding process of the other 

 cell. If, at the same time, the internal cell- membrane 

 remains connected with the outer cell-wall, the halves of 

 the dehiscent cell-membrane mostly remain approximated 

 together and connected with the uniting canal and the 

 spore, which usually fills this up (e. g. in Closterium 

 acerosum, lineatum, &c.) ; but if the loosened internal 

 cell-membrane gradually extricates itself from the outer, 

 the halves of the latter become separated and are readily 

 thrown off (e. g. in Tetmemorus). 



a. Conjugation in a parallel position. This is peculiar 

 to the group of Closterina. In the species with curved 

 cells the union takes place (with rare exceptions) by the 

 convex side, so that the conjugated specimens have their 

 points diverging. 



a. The halves of the dehiscent cells separate from each 

 other all round, the internal cell-membrane entering into 

 conjugation gradually emerging wholly or almost entirely. 

 The spore formed from the contents lies loose in the 

 delicate conjugation-cell. This is the case most evidently 

 in Tetmemorus ;* according to Ralfs figures, Docidium,^ 

 and a section of the Closteria having longitudinally 



* Ralfs, 1. c., p. 146, t. 24, f. 

 t Ibid., t. 26, f. 4. 



