DESMIDIACE* 



583 



do not usually acquire the full size or the characteristic markings of 

 the original before the division occurs) occupying the intermediate 

 place. At last the central fission becomes complete, and two bi- 

 partite fronds are formed, each having one old and one young seg- 

 ment ; the young segment, however, soon acquires the full size and 

 characteristic aspect of the old one ; and the same process, the 

 whole of which may take place within twenty-four hours, is repeated 

 ere long. The same general plan is followed in Micrasterias ; but 

 as the small hyaline hemisphere, put forth in the first instance from 

 each half-cell (fig. 437,, A), enlarges with the flowing in of the endo- 



D 



4|yj||fcL 



FIG. 437. Successive stages of binary subdivision of Micrasterias deMiculata. 



chrome, 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 distinguish- 

 able from the older half by its smaller size. The whole of this 

 process may take place within three hours and a half. In 

 Xphcerozosma the cells thus produced remain connected in rows 

 within a gelatinous sheath, like those of Desmidium (fig. 440) ; 

 and different stages of the process may commonly be observed in 

 the different parts of any one of the filaments thus formed. In any 



