300 THE PHENOMENON OF 



tion of the Diatomacese, and in itself presents nothing 

 impossible. The transition from a still incomplete 

 division to conjugation, is quite as conceivable as the 

 advance from a first incomplete division to a second, 

 such as we have become acquainted with in the quar- 

 tering of cells (p. 254). I will mention one more case 

 here, which can scarcely be explained otherwise than as a 

 reunion of two cells which have begun to divide, namely, 

 the not very rare abnormal occurrence of division of 

 individuals into three parts by two constrictions, in 

 .Euastrum and allied Desmidiacese. Nageli has figured 

 such a specimen of Euastrum {Cosmarium) Meneghinii, 

 Breb.* The formation of the spores of Spirogyra 

 mirabilis\ might be caused by a process similar to that 

 in Melosira, if that species really, as Hassall states, forms 

 spores in all the cells of filaments which do not enter 

 into conjugation with others ; this would be the more 

 conceivable in Spirogyra, since the external division of the 

 cells is preceded in this genus by the division of a 

 nucleus, and since the division is gradual. 



Lastly, the supposed conjugation of the contents of 

 two adjacent cells in the formation of the spores of 

 (Edogonium, and of Bulbochcste, with which it has a very 

 peculiar relationship, deserves an explanation. Decaisne| 

 and Hassall^ have noticed the strange circumstance, 

 that in the said two genera, the cell which immediately 

 precedes the swollen mother-cell of the spore, has little 

 or no green contents. Hassall thinks we may deduce 

 from this a passage of the contents of the lower cell into 

 the upper, and hence compares the process of spore- 

 formation of (Edogonium with that of the SpirogyrcB of 

 the section Bhynchokema, with the distinction, however. 



* Nageli, ' Einz. Algen.,' t. vii, f. 7, b, (as E. crenulatwm, Ehrenb. ?) 



t Zygnema mirabile, Hassall, 1. c, p. 156, t. 35. 



X 'Essai sur une Classification des Algues,' (1842,) p. 39. The figure 

 given at t. 14, f. 5, belongs to Bwlhoehate spharocarpa, A. Br., {setigera, 

 Auct., ex. p.) 



5 L. c., pp. 180—194, t. 50—53, {(Edogonium; t. 54, {Bulboch^te.) 



