74 Transactions of the Royal Microscopical Society. 
they are, as a rule, persistent throughout the entire life-cycle of 
every half-frustule that has undergone the allotted number of 
divisions. Moreover, I doubt the possibility of the persistence 
of the “ hoop” being demonstrable at all, even were we assured of 
its being a fact. But, fortunately, the determination of this point 
is quite immaterial in reference to the hypothesis with which 
Dr. Macdonald associates it. 
His hypothesis hinges (if I interpret it aright) on the validity 
of the assumption that the internal sectional] area of the valve, and 
the hoop belonging to it, are identica].* My impression is that 
their identity, in the sense implied, of absolute continuity, is by no 
means a characteristic of the diatom structure. 
With the utmost respect for Dr. Macdonald’s opinions, it appears 
to me that he has based his inferences, in this instance, far too 
exclusively on data supplied by a single group of diatoms; and 
that, even as regards that group, he has scarcely apprehended the 
structure correctly. It is true that Biddulphia and Isthmia, two 
genera specially examined by him as he tells us (loc. cit. p. 1), 
were amongst those singled out in my paper as offering the 
greatest facilities for the detection of differences in size between 
the older and younger valves of each frustule, and for showing 
most readily and distinctly the overlapping telescope-tube action of 
the free ends of the connecting zones; the entire frustules being 
often of such magnitude as to enable us to distinguish the contrast 
in the external diameters of the two halves arising from the con- 
ditions described. But I stated with reference to Isthmia—and 
this, let me observe, is a most important fact in relation to the 
argument—that, “although we sometimes meet in that genus with 
great variation in the length and breadth of the frustules growing 
on the same object” (say an alga), “it would be found that these 
marked differences do not always occur in the same filament, but in 
' separate filaments; and that frequently the primary frustule, or 
rather that valve of the primary frustule by which adhesion is 
secured, does not exceed in size the smallest of any of the neigh- 
bouring terminal frustules.” T 
But, as my entire remarks at every turn testify, I neither sus- 
pected nor did I assume the existence of an inviolable order of 
decrease such as that propounded by Dr. Macdonald; and for the 
best of reasons, namely, that the connecting zone of the primary 
valve in Biddulphia pulchella and Amphitetras antediluviana so 
often exceeds, in its diameters, the diameters of the valve itself, as 
to leave no room for doubt that a secondary or new half-frustule, 
* See the passage just quoted, in which Dr. Macdonald describes the perfect 
eaty in substance, and in outline, of the connecting zone and its parent 
t “On the Structure and Development of the Diatom Valve,” by G. C. 
Wallich, M.D., ‘ Quart. Journ. Micros. Science,’ December 1859. 
