16 RALFS, ON DIATOMACES. 
For several years I have attentively watched the circum- 
stances connected with the formation of these ner cells in 
Himantidium undulatum, by gathering specimens at short 
intervals. During great part of the winter, the filaments 
increase in bulk, by repeated division of the frustules, until 
they form large masses, filling the ditches; at length the 
inner cells make their appearance, at first sparingly, but as_ 
spring advances, it is difficult, in many situations, to obtain 
a filament without them. I have found that when these 
become abundant, the filaments cease to grow, and the entire 
mass soon breaks up and disappears. The same thing hap- 
pens in the other species of Himantidium, and in Meridion. 
I do not find that the inner cell commences in the centre, 
and pushes its valves outwards, as stated by Professor Smith. 
Vere this the case, the internal matter also would necessarily 
be pushed outwards by the advancing valves, and thus con- 
densed between them and the walls of the frustule. On the 
contrary, in the Himantidium the internal matter, before 
nearly fluid, collects within the new cell, becomes dense and 
more granular, and the new walls are formed round it in the 
situation they are to occupy, leaving an empty space between 
them and the walls of the frustule. 
The alteration and condensation of the colourmg matter, 
and the appearance, or at least great increase of vesicles, have 
a strong resemblance to what takes place previous to the 
formation of sporangia, the completion of which, as in this 
case, usually preludes the death and disappearance of the mass. 
As in most acknowledged sporangia, the cell thus formed 
always tends to assume an oval or orbicular form. It, how- 
ever, is very frequently, and perhaps generally, divided in 
halves, as in the fission of the frustules, so that the oval 
seems made up of two neighbouring frustules ; but this is not 
the case, as may readily be ascertained by noticing the mar- 
ginal puncta of the original frustule. 
Do these newly constituted cells ever continue to divide, 
as Professor Smith supposes? I believe not; at least I have 
never seen a specimen in which the semi-elliptie portions 
were separated by the interposition of other valves resembling 
either themselves or those of the ordinary frustule. For my . 
own part, I have been unable to trace the species after the 
formation of these cells, owing to the quickly succeeding 
disappearance of the mass. If, indeed, this renewed division 
does occur, the resemblance to what takes place in the 
sporangia of some species of Melosira would be increased. 
Professor Smith, in his most interesting and valuable 
account of the ‘Reproduction in the Diatomacez,’ enume- 
