REJUVENESCENCE IN NATURE. 139 



examined.* In the three genera named there is formed, 

 in each cell of the branches, after their last vegetative 

 division, an active germ-cell, vphich, furnished with four 

 cilia and a parietal red vesicle, breaks through the side 

 of the cell-wall in swarming out. In Stigeoclonium 

 protensum, K. (?), however, I have found, towards the 

 end of the period in which this plant appears, a modifi- 

 cation of the mode of formation of the active gonidia, 

 in which each cell gave birth not to one, but (by per- 

 pendicular division of the contents) two to four smaller, 

 more roundish swarmers ; as to the germinative powers 

 of which, however, I have no experience. In Braparnaldia 

 mutabilis, also, I observed an analogous formation of 

 microgonidia, not produced by perpendicular, but by 

 further continued horizontal division of the cells. 

 Nagelif mentions an observation on Melosira varians, 

 according to which the parietal colour-vesicles of this 

 Diatomacean sometimes become detached, and move in 

 the manner of swarm-cells, which might indicate an 

 occurrence of active microgonidia, together with the above- 

 described formation of larger, motionless germ-cells. 

 Many examples may be cited of the conjoined occurrence 

 of gonidia and true spores; many of these, however, 

 still require more accurate investigation. In the family 

 just named, the Diatomacese, certain genera seem to 

 exhibit, as a second mode of reproduction, a formation of 

 seed-cells with numerous young individuals, such as we 

 have assumed above of Closterium; so also Schizonema and 

 Micromega.% In the Desmidiacese, on the contrary, a 



* Vide Treviranus, 'Verm. Scliriften,' ii, 1, p. 73 (1817 — Draparnaldia) ; 

 Kiitzing 'iiber die Verwandluug der Lifusorien in niedere Algen-formen' 

 (18i4 — Stigeoclonmm stellare) ; Fresenius, ' Zur Coutroverse iiber die Ver- 

 wandlung von Infusorien in Algen' (1847 — CJuBtophora). I have myself 

 observed the active germ-oells of Draparnaldia mtitabilis, Sligeuclonium 

 thermale (June 1847), sitbspinosum (May 1847^, protenstim ? (May 1848), 

 temie (July 1849), Ch<stophora tuberculata (Aug. 1847), and elegans (Sept. 

 1847). 



■)■ Vide Nageli, ' Gatt. einzelliger Algen,' p. 10. 



% Vide Kiitzing, 'die Kieselsohaligen Bacillarien oder Diatomaceen' 

 (1844), t. X.XV, f . 8 ; t. xxvii, f. 11 ; t. xxviii, f. 1. 



