266 



THE MICROSCOPE AND ITS REVELATIONS. 



long. 1 The same general plan is followed in Micrasterias denticulata 

 (Fig. 157); but as the small hyaline hemisphere, put forth in the first 

 instance from each frustule (A), enlarges with the flowing in of the 

 endochrome, it undergoes progressive subdivision at its edges, first into 

 three lobes (B), then into five (c), then into seven (D), then into 

 thirteen (E), and finally at the time of its separation (F), acquires the 

 characteristic notched outline of its type, being only distinguishable from 

 the older half by its smaller size. The whole of this process may take 

 place within three hours and a half. 2 In Splicer ozosma, the cells thus 



Successive stages of Binary subdivision of Micrasterias denticulata. 



produced remain connected in rows within a gelatinous sheath, like those 

 of Didymoprium (Fig. 160); and different stages of the process may 

 commonly be observed in the different parts of any one of the filaments 

 thus formed. In any such filament, it is obvious that the two oldest 

 segments are found at its opposite extremities, and that each subdivision 

 of the intermediate cells must carry them farther and farther from each 

 other. This is a very different mode of increase from that of the Con- 

 fervacece, in which the terminal cell alone undergoes subdivision ( 52), 

 and is consequently the one last formed. 



1 See the observations of Mrs. Herbert Thomas on Cosmarium margariti- 

 ferum, in "Transact, of Microsc. Society," N.S., Vol. iii. (1855), pp. 33-36. 

 Several varieties in the mode of subdivision are described in this short record of 

 long-continued observations, as of occasional occurrence. 



2 SeeLobb in "Transact, of Microsc. Society," N.S., Vol. ix. (1861), p. 1 



