OF THE DIATOilE^. 65 



gation-cell, come a series of observations in which two new cells were seen 

 between the empty conjugated mother- cells, without any convincing evidence 

 being offered of a division of the mother-cells having occurred just before 

 conjugation, as in the cases hereafter to be mentioned, — where, rather, the 

 position of the empty cells in relation to the conjugation- cells, and the affinity 

 of the forms in question to some in which the entire development has been 

 obsei^ed, render it probable that the unicellular condition of the conjugation- 

 cell has hitherto escaped obsei'vation. In tliis group are to be counted Coc- 

 conema lanceolatum, C. Cistula, GompJionema dichotomum, G. lanceolatum, G. 

 marinum, AchnantJies longipes, Rhahdonema arcuatum, ColUtonema suhcohcerens, 



" In a smaller number of DiatomefB, species of the genera so nearly allied 

 together, Epithemia, Ci/mbeUa, and Amj^hora, the conjugation is immediately 

 preceded by a division of the mother-cells into two, analogous to the division 

 of the ceUs of CJosterium rostmtum when about to conjugate. This division 

 is longitudinal, taking place exactly as in the vegetative division in Cymhella 

 Pedicidus, Ampliora ovalis, arid Epithemia Sore.v, but transverse and in a 

 direction crossing that of the vegetative division in Epithemia turyida, E. gihha, 

 and E. verrucosa. 



" Eecent obsei^ations show distinctly that the conjugation of the Diatomese 

 agrees in all essential points with that of the Desmidieae. ^yhen a cell is 

 about to conjugate, there is produced in it a coat round the entire contents, 

 accm-ately liiiing the old membrane, but not adhering to it. . The growth of 

 this coat cracks the old cell-membrane exactly in the same way as occurs in 

 vegetative division. From the fissure the young, smooth coat emerges, in the 

 form of a vesicle, and unites with the similar structiu"e produced by a neigh- 

 boiuing cell. Al. Braun thought it must be assumed, from Thwaites's obser- 

 vations, that the primordial utricles of the two conjugating Diatomean cells 

 imited ; but that this is not the case, and that a soft and flexible cell-mem- 

 brane, protnided from the cracked, rigid, old shell, encloses the contents 

 destined to be blended with those of the neighbouiing cell, is distinctly shown 

 by Smith's figure of Rhahdonema arcuatum, and Carter's of Cocconeis Pedi- 

 cidus and Amphora ovalis. The introductory part of the conjugation is dis- 

 tinguished in no respect from the vegetative cell-division in Epithemia Sorecc, 

 Amphora ovalis and Cymhella Pediculus, and, further, in CJosterium rostratum ; 

 in Epithemia turgida, gihha, and verrucosa, only by a different position of the 

 wall dividing the mother-cell ; in the rest of the Diatomese and Desmidieae, by 

 omission of the formation of septa, — frequently, also, by one-sided dehiscence 

 of the cracked mother-cell, whose shells remain still connected at one side. 



" Thwaites's observations estabhshed that the ceU produced from the conju- 

 gation of two cells of a Diatomacean, very soon after its origin, assumed the 

 form of the mother- cell, becoming distinguishable from it almost solely by 

 being twice as large. Smith has endeavoured to render it probable that the 

 colonies of young individuals, enclosed in a cyst, of Coccoyieis Cistida, Gom^ 

 phonemci dichotomum, and Synedra radians, some of which he found associated 

 with conjugated, fiill-grown individuals, must have originated from the divi- 

 sion of the spores (sporanges of Enghsh authors). This hypothesis has much 

 in its favoiu% but, in the present condition of our knowledge, it is inexplicable 

 where the sihcious shells of the spore-cells remain. However this may be, 

 there is no doubt of the occurrence of cysts of this kind. In the same pools 

 of a marshy meadow which repeatedly furnished me with conjugated indivi- 

 duals of Cyclotella late in autumn, I found, in early spring of two successive 

 years, globular cells, each of which enclosed a great number (32 to 40) of 

 small individuals of the same species. The walls of these cells appeared 

 shai^ply defined internally and externally ; the contents of a thin, fluid nature. 



