240 Proceedings of tJic Royal Iri>iJi Academy. 



push, the Talves of tho mother-cell more and more "widely asunder, 

 A ne^n" silicious valve is secreted hy each of the two masses, on the 

 side opposite to the original Yalves. And, Tvhen this process has heen 

 completed, two distinct frustiiles are foi-med, the silicious Talves in 

 each being one of the valves of the parent-cell, and a newly secreted 

 valve apposed to it. During the active life of the cell this process of 

 self -division is continued, and is rapidly completed. On this subject 

 Smith observes, " I have been unable to ascertain the time occupied in 

 a single act of self-division ; but, supposing it to be completed in 

 twenty-four hours, we should have as the progeny of a single frustule 

 the amazing number of one thousand millions in a single month — a 

 circumstance which will, in some degree, explain the sudden, or, at 

 least, rapid appearance of these organisms in localities where they 

 were, but a short time previously, either xinrecognised or sparingly 

 diifused." — British Diatomacese, vol. i., p. 25. 



It seems probable that the Diatomacese are sometimes reproduced 

 by zoospores. Eabenhorst records his having observed a specimen of 

 3Ielosira varians, in which, from the sporangial fiaistule, there issued 

 what appeared to be geiTas, and has described the process. Die 

 Susswasser Diatomaceen, T. x., fig. 18 c. A similar occurrence was 

 noticed by myseK in 1858, in the case of Pleurosigma Spencerii : and 

 Castracane has recorded two or three observations of the same kincL 

 So far as I am aware, the development of these zoospores, if such they 

 be, has in no case been traced through its successive stages to its- 

 rdtimate result ; but there is nothing unreasonable in the presumption 

 that the phenomenon may be a phase of the reproductive process. 



Another mode of reproduction in the Diatomacese is by conjugation,, 

 of which, according to Smith, there are four distinct phases. First. The 

 union of two parent frustules issues in the formation of two sporangia. 

 Second. Two parent fi'ustules produce only one sporangium. Third. A 

 single fiTistule develops a single sporangium. Fourth. A single parent 

 fiTistule produces two sporangia. In the first stage of the conjugative 

 process, a mucous sac is secreted by the parent frustules, within 

 which the sporangia are developed ; these sporangia, in some cases, 

 lie parallel, and in other species at an angle with the parent frustules 

 or valves, as the case may be. A phase of conjugation, quite distinct 

 fi-om the four just referred to, came under my notice, many years ago, 

 in the case of Diatoma vulgare.'^-' I observed numerous instances of the 

 long chain of concatenated fi'ustules in theii' normal condition with a 

 sudden jerk fold themselves into a solid mass. In a very brief period 

 a mucous sac was seen to develop itself, inclosing the whole mass of 

 frustules, and in some cases enveloping forms of a different species 

 which happened to be in immediate proximity. By degrees the mu- 

 cous sac pushed itself forward, sometimes in a single projection, some- 

 times in two, and into these prolongations the cell-contents of the 



* Xatiiral Ilistoiy Eevio-n-, 1859. vol. vl., page TO, PL ix. 



