243 REV. B. o'meara. 



frustule had been to some extent recognised, its true structure 

 and its relation to the development of the plant were over- 

 looked. 



Wallich, in 1858, observed that in the case of Amphiteh'as 

 Triceratium and Biddulphia, " the hoojj consisted of two 

 pieces, Avhich at first entirely overlap each other, but as the 

 process advances, recede from each other, and whilst so 

 receding, appear like three distinct parallel annuli, the centre 

 being less diaphanous, and its markings more confused, in 

 consequence of its being in reality the overlapping and double 

 portion referred to." Wallich still further contributed to 

 establish a true conception of the mode of growth in the 

 hoop, by proving that in the families above specified, " the 

 growth of each plate of the connecting membrane takes place 

 at the margin furthest off from the valve to which it is 

 attached." Pfitzer endorses the accuracy of Wallich's views, 

 and expresses an opinion that greater value attaches to this 

 discovery than even to that of the bivalve character of the 

 frustule. 



Presuming the separate plates of the connecting membrane 

 to be in all cases as Wallich describes them, '' marginal ex- 

 tensions of the valves perpendicular to their general plane," 

 and that the formation of the new valves takes place within 

 it, the effect of these conditions on the growth of the future 

 frustules must be obviously to effect a gradual diminution in 

 the size of the valve equivalent to the thickness of the 

 siliceous plate of the hoop. 



In the year 1869, Dr. M'Donald, in his interesting paper 

 " On the Structure of the Diatomaceous Frustule and its 

 Genetic Cycle" ('Ann. and Mag. of Nat. Hist.,' 4 ser., 

 vol. iii, 1869, p. 1), adopts the theory of Wallich concerning 

 the structure of the hoop, and traces out its effects on the de- 

 velopment of the frustule. '^ As each perfect frustule consists 

 of an older and a younger valve, never of two valves of the 

 same age, Kiitzing's names — primary as applied to the former, 

 and secondary to designate the latter, or the invaginated 

 valve — can be open to no possible objection. But to these 

 it is absolutely necessary to add two tertiary valves of the 

 same age, resulting from the process of fission, viz. the first 

 tertiary developed in connection with the primary valve, and 

 the second tertiary forming a new frustule with the secondary 

 valve." 



The process of diminution in each successive frustule goes 

 on until the minimum is reached, when the frustules conju- 

 gate, and from this operation arises a sporangial frustule, in 

 all respects similar to its parents, but double the size. 



