EDWARD M. NELSON ON DIATOM STRUCTURE. 87 



have a still larger number, and so on until we find the circular 

 primaries compressed into hexagons. It is important to note 

 that this compression into hexagons commences at the periphery 

 of the valve, the last primaries to retain the circular form being 

 situated at the centre of the valve. This is strictly in accordance 

 with the law of diatomic growth I have previously enunciated,* 

 viz., mature at the periphery, immature at the centre. 



Although these forms have specific names allotted to them, 

 I regard them as being early stages in the evolution of the 

 Aster omphalus. 



With regard to the eye-spot layer, it is altogether absent in 

 the first stages, but makes its appearance when the primaries 

 assume their hexagonal or polygonal shape. The terminus ad 

 quern may be said to be a circular plate of silex sparsely perforated 

 with circular primaries. 



* Q.M.C. Jonrn., Vol. 3, Ser. 2, pp. 201 and 308 (1888). 



