REJUVENESCENCE IN NATURE. 287 



the external membrane ; the reproductive-cell is formed 

 through contraction of the contents clothed by the internal 

 cell-membrane. The formation of the reproductive-cell 

 (in the cases to be mentioned, of a spore) is consequently 

 here not a direct result of the conjugation, but it is 

 formed subsequently in the interior of the conjugation- 

 cell, in the strongly expanded isthmus of this. The deli- . 

 cate internal membrane, with the contents enclosed by it, 

 drawing itself out of the (on account of the thickness of 

 the outer membrane, rigid) extremities of the double- 

 cell, forms a seed-cell, at first cruciate-four-lobed, then 

 bluntly quadrangular, and finally globular, clothed by 

 a many-layered thickened membrane, within the persistent 

 four-horned conjugation -cell. From Ralfs's representation 

 this is most probably the way in which the process is to 

 be understood in Cylindrocystis Brebissonii, Menegh. 

 (ex. p.)* The participation of the outer membrane in 

 the conjugation is beyond doubt ; this is the principal 

 character distinguishing the genus Cylindrocystis from 

 Penium. The resemblance of the quadrangular internal 

 cell to that of Tetmemorus, in which this is undoubtedly 

 formed of a delicate cell-membrane, testifies in favour of 

 the existence of the latter in this case also. From the 

 cruciate or quadrangular shape of the spore, it may be 

 conjectured that the conjugation of genus Stauroceras,^ 

 likewise belonging to the group of Closterina, as also 

 that of the Zygnemaceous genus Staurocarpus,\ Hassall 

 (Staurospermum, Kg.), belong here. 



5. The conjugating cells unite with participation of the 

 outer membrane ; the reproductive-cell is formed out of 

 the mere contents, as a new cell, inside the mother-cell. 

 This case (occurring in the majority of the Zygne- 

 macese) differs from the preceding by the non-participation 



* Penium (Cylindrocystis) Brebissonii, Ralfs, 'British Desmidiese, 5 p. 153, 

 t. xxv, f. 6. 



f Ehrenberg, ' Infusionsth.,' tab. vi, f. 94. Ralfs, 1. c., t. xxx, f. 4, 

 Stauroceras subulata, K. ;) ibid., f. 3, (St. Acus, K. ;) f. 5, (St. acutum, Breb.) 



J Hassall, ' British Freshwater Alga;,' t. 4749. 



