Bibliographical Notices. 139 



aids to enable us to trace the changes and satisfy ourselves of their 

 real identity with that already described ; and I hope to be able to 

 show that this identity does exist. 



" In Nostoc and Anabaina the cells are globular, and as there is no 

 constriction we might remain ignorant of the real method of divi- 

 sion ; but, guided by the analogical process in the Desmidiece, I hope 

 to make it sufficiently plain. The hemispheres are thrust apart by 

 the new formation ; but now it is the outer rounded margin that we 

 look to for an explanation. If a globe be cut into two equal portions, 

 each will represent half a circle. By comparison with the neigh- 

 bouring cells, we find that thesa two half circles remain unaltered, 

 and are merely separated from each other, for if again brought 

 together they would reconstitute the former globe. The new for- 

 mations however separate them further and further, until the inter- 

 vening space equals that occupied by the original globe, and then 

 we find two globes exactly like the primary one, the internal half of 

 each being the newly-formed one. During this time the inner por- 

 tions, as they extend, develope more and more of the circle, until 

 each becomes, as I have stated, a perfect hemisphere. The whole 

 process cannot, of course, be seen in the same cell ; but in a dividing 

 filament some joints may be observed in one stage and some in 

 another, which renders the evidence complete. 



" When the cell is oblong, or only rounded at the extremities, the 

 process, though similar, is less evident : the cell at first seems 

 merely to elongate until it obtains nearly twice its original length, 

 when the division commences and the rounding of the new ends 

 becomes apparent. The tapering cell presents but little difference, 

 for the separation takes place before its extremities are fully deve- 

 loped. Sometimes these cells separate obliquely, as in Spirotcenia 

 and Scenedesmus. 



" I ought to state however that the opinions advocated above do 

 not agree with those of M. De Brebisson, who has attained so high 

 a reputation for his intimate acquaintance with the freshwater Alga?, 

 and to whose kindness I have been so often indebted during the 

 progress of the present work. He considers that there is an essen- 

 tial distinction in the mode of division between the Desmidiece and 

 Nostochinece (including in the latter the Palmellece), and that from it 

 indeed differential characters are obtained by which we can distin- 

 guish these nearly-allied groups. He observes of Hormospora mu- 

 tabilis, Breb.*, ' lis sont le plus souvent gemines, se multipliant par 

 une division spontanee (deduplication) transversale, comme cela 

 arrive dans quelques autres Pleurococcoidees. Une division ana- 

 logue a lieu dans les Desmidiees, auxquelles on serait d'abord tente 

 de rapporter les Hormospora ; mais les demi-corpuscules (hemiso- 

 mates) des Desmidiees developpent a leur point de separation une 

 nouvelle portion semblable a la premiere, tandis que, dans l'accrois- 

 sement des Nostocinees, les corpuscules sont divises en deux par un 

 etranglement transversal, sans qu'il s'ensuive une reproduction sur 



* Annates des Sciences Naturclles, Jan. 1841. 



