BINARY SUBDIVISION OF DIATOMACEiE. 



283 



sists in the elongation of the cell, and the increase in the breadth 

 of the ' hoop,' which is well seen in Fig. 121, a ; for in the newly- 

 formed cell e, the two valves are in immediate apposition, in d a 

 hoop intervenes, in a this hoop has become much wider, and in 

 b the increase has gone-on until the original form of the cell is 

 completely changed. At the same time, the endochrome separates 

 into two halves, so that its granules form two layers applied to the 

 opposite sides of the frustule ; the nucleus also subdivides, in the 



Fig. 121. 



Biddulphia pulchella : — a, chain of cells in different states : a, 

 full size ; b, elongating preparatory to subdivision ; c, formation 

 of two new cells ; cl, e, young cells ; — B, end-view ; — c, side-view 

 of a cell more highly magnified. 



manner formerly shown (Plate vin.,fig. 1, a, h, i) ; and (although 

 the process has not been clearly made -out in this group) it may be 

 pretty certainly concluded that the primordial utricle folds-in, first 

 forming a mere constriction, then an hour-glass contraction, and 

 finally a complete double partition, as in other instances (§249). 

 From each of these two surfaces a new siliceous valve is formed, as 



