Cell-division 



105 



altogether true, as it has been shown in some of the filamentous species, and 

 is possibly the case in many other solitary forms, that daughter-cells are often 

 produced of larger size than the parent-cells. Such individuals are recognized 

 by the thickened rim of the valves. It is very probable that in many cases 

 the new valves are only feebly silicified until they have been extruded beyond 

 the connecting-band of the old girdle. They are thus capable of slight 

 extension before their size becomes fixed by strong silicification. Such an 

 increase in the size of the valves, however slight, would have a retarding 

 influence on the diminution in size of the cells, the reduction in size not 



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Fig. 76. Four stages in the cell-division of Navicula ( Pinnularia) oblong a Kiitz. The pair of 

 conspicuous rounded bodies in each daughter-cell of the right-hand figure are Biitschli's red 

 corpuscles. (After Lauterborn. ) 



being proportionate to the number of bipartitions. In the genus Melosira, 

 O. Miiller has shown that multiplication of the cells takes place in such 

 a manner as to prevent as much as possible the division of the smaller 

 daughter-cells. 



Allen and Nelson ('00) have shown by series of pure cultures that cell- 

 division in Nitzschia Closterium forma minutissima resulted in no appreciable 

 reduction in size even though the number of generations in two years was 

 incalculably great. 



