The Development, dc., of Diatomacee. By Dr. G.C. Wallich. 69 
developed, whilst in others it is not so. But, from the invariable 
comparative obscurity of the more delicate markings in all, until 
the siliceous surface 1s freed from its soft covering by boiling acids, 
I inferred that all extra-frustular structure is secreted by the 
“ primordial utricle” through the marginal apertures of the valves, 
much in the same way that the epiderm of the molluscous shell is 
secreted by the margin of the animal’s “ mantle.” Of its highly 
elastic nature there is abundant evidence, and from its retaining its 
character unimpaired, it is impossible to doubt its vitality ; and we 
are thus furnished with presumptive proof that the invisibility of 
the motile and prehensile filaments, whose existence I endeavoured 
to demonstrate inferentially, must be due to the same causes that 
enable the subtle gelatinous or membranous elastic film of some 
forms, as, for example, Bacillaria paradoxa, to defy our most 
perfect optical resources. * 
It will have been noticed that Dr. Macdonald fully and un- 
grudgingly confirmed, on the basis of his own independent observa- 
tions, all the leading facts and inferences mentioned in my papers 
of 1859 and 1860. Unfortunately, however, there happened to be 
two points, which, though almost trivial as touched on by me at 
the time I wrote, did not quite meet Dr. Macdonald’s concurrence 
when he published his paper of 1869; and these at once assume 
importance from the fact of his having referred to them in con- 
nection with an elaborate hypothesis propounded by him in rela- 
tion to the order of increase of the Diatomacee—an hypothesis 
which, if sound, must of necessity have shown me to be wrong on 
the points referred to; but, if unsound, would only lend them 
additional corroboration. 
To render the arguments more readily intelligible, I will at 
once state what the two issues really amount to on which Dr. 
Macdonald and myself seem so materially to differ. Put in the 
shape of questions, they are:—1. Is each connecting zone, as 
affirmed by Dr. Macdonald, “ not merely connected, but directly con- 
tinuous with the body of its own valve, and therefore essentially a 
persistent part of the valve”? Or is it, as I (in common with most 
other observers) believe, so far a supplementary portion of the valve 
as to become deciduous on some genera, although more or less per- 
sistent in others. 2. Are the extreme differences discernible in the 
size of the frustules of some species obtained from different locali- 
ties, or from the same locality in differing seasons, sufficiently and 
wholly accounted for by the successive diminutions in size resulting 
from division, as urged by Dr. Macdonald ? Or is there sufficient 
* For a detailed account of all the evidence on which these conclusions were 
arrived at, I beg to refer to a paper of mine, ‘On the Distribution and Habits of 
the Pelagic and Fresh-water Diatomacee,” published in the ‘Annals and Mag. 
Nat. Hist’ for January 1860. 
