HISTORY OF DIATOMS. 



varying thickness, which is generally considerable at the ends of those 

 diatoms whose axis is prolonged. The cellular membrane is transparent 

 and, as in all vegetable cells, is formed at the expense of the primi- 

 tive protoplasmic mass. It does not adhere intimately to the valves 

 because the use of even the weakest re-agents suffices to make it con- 

 tract by forcing its contents towards the centre of the frustule. 



The endochrome (fig. i CC) is of a golden or brownish yellow and 

 sometimes, but rarely, of a greenish colour, as in the case of Navicula 

 cuspidata. The endochrome is arranged either in granules (as in the cylin- 

 drical and discoid forms) or else in plates, which are now sometimes called 

 chromatophores. 



The chromatophores do not always form continuous bands ; they are 

 occasionally jagged and perforated in a most peculiar way, and this is the 

 case with many marine diatoms. 



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Fig. 2. 



Chromatophores of Marine Diatoms. 



A. Schematic figure of the portion of chromatophore resting on the superior valve of PL 



angulatum. B. The same of the inferior valve. C. The two parts combined, showing the 



appearance of the whole. D. Schematic figure of an exposed chromatophore of PI. Balticum. 



E. Median portion of the chromatophore of Nitzschia Sigma (after Otto Muller). 



