The Development. &c., of Diatomacer. By Dr.G C. Wallich. 63 
in a determinate order, according to the inherent tendency of the 
species; but that the ultimate figure of the markings is due to 
forces exerted upon the young valve, whilst in a yet plastic state 
and retained within the connecting zone of the parent valve.” 
“And lastly, that the variation in size, and in the degree 
of fineness or coarseness of the markings, is, within fixed limits, 
dependent on the conditions under which the Sporangial Frustule 
gives egress to the germs of the new generation; but that the 
ordinary process of division is, of itself, sufficient to bring about 
great variation when operating through a long succession of 
individuals.” 
In the course of my observations, I endeavoured to show why 
variation in the size and markings of different individuals of the 
same species is not only consistent with, but naturally follows 
from the evidence which was adduced. That whilst the total 
number of striae upon a valve may remain nearly uniform in every 
valve of the same species, the number of strize upon the fractional 
part of a valve (say the yooth part of an inch) admits of just as 
much variation as the size of the valve, and proceeds simultaneously 
with it during division, but not afterwards. That, in all likeli- 
hood, the internal dimensions of the connecting zone, by which the 
young or new valve is protected during its secretion and consolida- 
tion, determine the size to be attained by it; and although the 
valve may subsequently receive some additional siliceous deposit, 
the whole of its characters, as we see them in the microscope, are 
indelibly impressed upon it before or immediately after its libera- 
tion. And further, that each of the two connecting zones, which 
(as had been pointed out in my paper “On Triceratiwm”) enter 
envariably into the formation of every diatom frustule, increases in 
depth by secretion of fresh siliceous matter at its free margin only ; 
being “thus enabled to slide the one within the other, telescope- 
fashion, to accommodate itself, with its fellow, to the increase of 
the cell-contents during division ; this feature being most strikingly 
Se in such genera as Biddulphia, Amphitetras, Isthmia, and 
others.” 
In the observations now about to be made, and which form, as 
it were, a supplement to those just cited, I propose to bring 
together for critical examination any new facts and deductions on 
the subject that may have been since then published. I would, 
however, mention at the outset that although I still adhere, in 
every material respect, to my old convictions on the question 
of “markings,” it is neither my intention, nor does the scope of 
this paper demand that I should, on the present occasion, allude to 
it further than by indicating how far the markings are the result 
of constantly or inconstantly acting forces. 
The first thing to be done is to show clearly what are the 
F 2 
