MULTIPLICATION OF DESMIDIACE^. 269 



described (§ 150) ; and in this manner, out of the lowest cell of 

 the filament a, a double cell b is produced. But it will be ob- 

 served that each of the simple cells has a bifid wart-like projec- 

 tion of the cellulose coat on either side, and that the half of this 

 projection, which has been appropriated by each of the two new 

 cells, is itself becoming bifid, though not symmetrically ; in process 

 of time, however, the increased development of the sides of the 

 cells which remain in contiguity with each other, brings up the 

 smaller projections to the dimensions of the larger, and the 

 symmetry of the cell is restored. In Olosterium (Fig. 72, d), the 

 two halves of the endochrome first retreat from one another at 

 the middle line, and a constriction takes place round the cellu- 

 lose coat ; this constriction deepens until it becomes an hour- 

 glass contraction, which proceeds until the cellulose coat entirely 

 closes round the primordial utricle of the two segments ; in this 

 state, one half commonly remains passive, whilst the other has 

 a motion from side to side, which gradually becomes more 

 active ; and at last one segment quits the other with a sort of 

 jerk. At this time, a constriction is seen across the middle- of 

 the primordial utricle of each segment ; but there is still only a 

 single chamber, which is that belonging to one of the extremities 

 of the original entire frond. The globular circulation, for some 

 hours previously to subdivision, and for a few hours afterwards, 

 runs quite round the obtuse end a of the endochrome ; but 

 gradually a chamber is formed, like that at the opposite extre- 

 mity, by a separation between the cellulose coat and the primor- 

 dial utricle ; whilst at the same time, the obtuse form becomes 

 changed to a more elongated and contracted shape. Thus, in 

 five or six hours after the separation, the aspect of each extremity 

 becomes the same, and each half resembles the perfect frond in 

 whose self-division it originated; and the globular circulation 

 within the newly-formed chamber comes into connection with 

 the general circulation, some of the free particles which are 

 moving over the surface of the primordial utricle, being drawn 

 into its vortex and tossed about in its eddies. The process is 

 seen to be performed after nearly the same method in Stauras- 

 trum (Fig. 71, D, e) ; the division taking place across the central 

 constriction, and each half gradually acquiring the symmetry 

 of the original. In such forms as Cosmarium, however, in which 

 the cell consists of two lobes united together by a narrow isthmus 

 (Fig. 75), the division takes place after a difterent method ; for 

 the central region of the isthmus expands, and displays two 

 globular enlargements, separated from each other and from the 

 two halves of the original cell (which their interposition carries 

 apart) by a narrow neck; and these enlargements increase, until 

 they assume the appearance of the half-segments of the original 

 cell. In this state, therefore, the plant consists of a row of four 

 segments, lying end to end, the two old ones forming the ex- 

 tremes, and the two new ones (which do not usually acquire the 

 full size or the characteristic markings of the original, before the 



