240 Proceedings of the Royal Irish Academy. 
push the valves of the mother-cell more and more widely asunder. 
A new silicious valve is secreted by each of the two masses, on the 
side opposite to the original valves. And, when this process has been 
completed, two distinct frustules are formed, the silicious valves 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 unrecognised or sparingly 
diffused.’’—British Diatomacesze, vol. i., p. 25. 
It seems probable that the Diatomacesze are sometimes reproduced 
by zoospores. Rabenhorst records his having observed a specimen of 
Melosira varians, in which, from the sporangial frustule, there issued 
what appeared to be germs, and has described the process. Die 
Susswasser Diatomaceen, T. x., fig. 18 ¢. A similar occurrence was 
noticed by myself in 1858, in the case of Pleurosigma Spencerii: and 
Castracane has recorded two or three observations of the same kind. 
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 
ultimate 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 Diatomacez 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 frustules produce only one sporangium. Third.’ A 
single frustule develops a single sporangium. Fourth. A single parent 
frustule 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 
from 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 frustules in their 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 
* Natural History Review, 1859, vol. vi., page £0, Pl. ix. 
