OF THE DIATOilEJE. bM 



another. Close investigation shows that the broader empty coats have an 

 orifice, towards the border of which the membrane grows gradually thinner. 

 These holes have not the aspect of perforations of the outer walls through 

 external injiuy ; they rather resemble the orifices of the walls of CJadoj^hora, 

 thi'ough which the swarming-spores escape. It might be conjectured that 

 the plant multiplied by swarming-spores, and that solitaiy ones becoming 

 developed inside the empty coat of the mother- cell gave rise to that appear- 

 ance ; but this is contradicted by the great frequency of their occurrence, 

 as also by the circumstance that we never find a number of green cells inside 

 one cell- coat. It is more probable that the contents of the cell contract, and 

 become coated with a new membrane, when the old one is perforated, — by 

 unknown causes, which perhaps lie in the course of development of the 

 species. 



" If we seek to bring the phenomena introductory to vegetative cell-mul- 

 tiplication under one point of view mth the preparations for conjugation, we 

 find that, in the Desmicheae, in both cases a new membrane is formed aroimd 

 the total contents of the ceU, wliich indeed lies close upon the old coat at all 

 points, but by no means adheres to it, as we are accustomed to conceive of 

 the so-called layers of thickening of the cell-wall. The growth of the young 

 membrane cracks the stronger old one — in vegetative cell-multiplication 

 always in an annular fonn, in conjugation, mostly in a one-sided manner, 

 with a valve-hke slit (Hi/ahtheca dissiliens ; Closterhim). At this stage 

 first occurs a distinction between the two processes of development, — the 

 foimation of a sej)tLun taking place in cell-division, while in conjugation the 

 protniding part of the young membrane continues to enlarge outwards, 

 without, in many cases, any separation of the contents into two halves taldng 

 place. The younger, innermost layer of membrane remains with that portion 

 lining the old cell-coat, sticking wholly in this in Hyalotheea, Bambimna, 

 Cosmarium. But even in individuals of species of the last genus it sometimes 

 occurs, in Tetmemorus and Closterium (e. g. C. acutum) as a rule (although by 

 no means without exception), that the ends of the connected inner coats of 

 the conjugating cells draw themselves out of the cast-off" shells of the mother- 

 ceUs, in extreme cases entirely ; so that the cell originating by the blending 

 of the internal coats of two individuals (inside which the spore is fonned) 

 becomes capable of being rounded off" into a sphere. 



" Both the ceU- division and the preparation for conjugation of Zygnemese 

 are distinguished from the processes in Desmidiea? by the circumstance that 

 in the former the wall of the oldest cells grows in its entire mass, and does 

 not allow the younger layers of membrane to protnide through fissures or 

 slits. 



" In the Diatomece, lastly, the division into two, like the conjugation, takes 

 place, seemingly, in all cases, through and after a preparatory contraction of 

 the contents or separate portions of the contents of the cells ; and in not a 

 few cases the conjugation takes place during, and is accompanied by, di\-ision 

 of the contracted contents into two portions. A\Tiat import for the life of the 

 species has the conjugation of the Zygnemeae, Desmidieae, Palmelle£e (Pal- 

 mofjloea), and Desmidie^ ? Our knowledge of the race of Algae, so import- 

 antly advanced by the labours of Pringsheim and Cohn, should allow a more 

 positive answer to this question than that inquirer, to whom the study owes 

 most brilliant acquisitions, is inclined to give. The idea of sexuality of the 

 lower Algae depends principally upon the perfectly justifiable, but still only 

 analogical conclusions which, starting from the observations made diuing a 

 centurj^ on the Phanerogamia, have advanced, through the intermediation of 

 those, less numerous, on the Vascular Ci-jiitogamia and Muscineae, and the 



