Diatomaceous Frustule^ and its Genetic Cycle. 3 



' hoop.' " This description would lead one to suppose that the 

 hoop was a single and distinct segment, serving to connect 

 those portions of the frnstule to which the term valves is more 

 particularly confined, and that the growth of the hoop was, 

 therefore, not limited to one border more than the other. 



Dr. Wallich, I think, very successfully refutes the idea of a 

 continuous growth of the Diatomaceous frustule, the fact being, 

 as he states, that " the variation in the size of the valve and 

 the number of its strias proceed ^^ar^ passu during the process 

 of division, but not subsequently." He admits that "growth 

 may take place to the extent of new siliceous matter being 

 secreted along the margins of the valve, its depth being thereby 

 augmented;" but he considers it highly probable " that the 

 connecting zone by which the young valve is protected dmnng 

 its secretion and consolidation determines the limit of the di- 

 mensions to be attained by it." He states, moreover, that 

 " in truth no two valves of the same frustule can be of the 

 same size, for the new valves, being formed within the ' con- 

 necting zones ' of the parent frustule, must be smaller than 

 these." This, I should think, is the essential cause of the 

 great diversity of size observable in frustules of the same 

 species being constant and universal ; but he lays more stress 

 upon the peculiar idiosyncrasy of the sporangial frustule, 

 vicissitudes of climate, and increased or diminished som'ces of 

 nutritive matter. Notwithstanding all the above important 

 facts and deductions, in common with other authors Dr. 

 Wallich seems to consider the hoops of the " connecting zone " 

 quite supplementary, and not essentially persistent parts of 

 the valves themselves, though often easily separable. 



In the ' Micrographic Dictionary,' at p. 201, we read, " The 

 hoop appears to be a provision for the protection of the nascent 

 half-frustules, which probably do not become silicified until 

 full-grown, and would thus be liable to be injm-ed or disturbed 

 by the movements of the rigid and heavy parent half-frastules 

 if the centre of the frustule in process of division was naked, 

 as in the Desmidiaceaj." In all this the existence of two dis- 

 tinct layers in the " hoop " is not even hinted at, nor their 

 identity each with its own valve at the true ends of the 

 frustule. 



It stands to reason that as the two new half-frustules are 

 developed endogenously, or within their parents, the former 

 must be smaller than the latter by the whole thickness of the 

 siliceous investment ; and this will continue to be the case 

 gradatim in the direct line of descent, though of course aU the 

 pullulations successively taking place in the same half-frustule 

 will be uniformly of the same size, holding the relation of cast 



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