78 Transactions of the Royal Microscopical Society. 
as an outgrowth from the parent segment. In all my experience 
I have never seen a new half-frustule so developed; nor do I 
believe such an occurrence to be possible ; for, from the very outset, 
there is a ‘‘suture line,” and this suture line is nothing more or 
less than the embryo connecting zone, which is not a subsequent 
interpolation, as it were, but a simultaneous product with the 
development of the new valve itself. And, if so, whenever the 
parent connecting zone exceeds its own valve in diameter (as I 
have shown to be the case frequently in some diatoms), there 1s, of 
course, an end to Dr. Macdonald’s deduction from the premisses, 
namely, “that the newly formed half-frustule must be smaller than 
the parent valve, by the whole thickness of the siliceous invest- 
ment, and that this will continue to be the case gradatim in the 
direct line of descent, through the course of all the pullulations 
taking place successively in the same half-frustule.” 
Should further proof be desired of the impossibility of the 
parent valve forming a “mould” within which the new valve is, 
as it were, cast, it will I think be found in the significant fact 
that in several well-known genera of Diatoms, the two valves con- 
stituting each frustule are never symmetrical, eacept in sectional 
outline where each margin is in connection with its own connecting 
zone; the symmetry in this region being obviously essential to 
admit of one of the connecting zones slipping accurately (as before 
described) over the other. We are thus furnished with what 
appears to me to be conclusive evidence that there resides in the 
endochrome of these organisms, an inherent and hereditable forma- 
tive power of a determinate kind, though by no means so deter- 
minate as the force which governs the formation of every crystal. 
At the same time it appears certain that accidentally caused devia- 
tion from the normal form may be handed down, in direct descent, 
from the frustule in which it first originated. But it must be 
borne in mind that this does not take place unless the mechanical 
or other cause has acted on the connecting zone. Thus it not 
unfrequently happens that a sinuosity, or a notch, may be found 
in a large number of specimens of a species of diatom collected in 
one piece of water, whereas not a single example of it is to be met 
with in specimens collected in another piece of water in the same 
neighbourhood. 
Repropuction.—In the Introduction to the second volume 
of ‘The Synopsis of the British Diatomacex, the late lamented 
author of that work prefaced a brief account of the “ sporangial 
frustules” of certain species with the characteristically candid 
admission that, during the three years which elapsed since the 
publication of the former volume, no new light had been thrown 
on the subject of reproduction, and that the entire process was, in 
short, “ too imperfectly understood.” 
