66 Transactions of the Royal Microscopical Society. 
structure, and still more important, though certainly far less intelli- 
gible, vital processes in these organisms, with a view to show how 
singularly vague and unsatisfactory these opinions are. For the 
purposes of my argument, it is necessary that I should now, for a 
time, invite attention to the observations of a distinguished 
naturalist, whose judgment on questions of this kind, based as 
it invariably is on patient and conscientious research, must always 
command respect, even when it does not happen to be altogether in 
unison with one’s own opinions. 
In a paper “On the Diatomaceous Frustule and its Genetic 
Cycle” (‘ Annals and Mag. Nat. Hist.’ January 1869), Dr. J. Denis 
Macdonald wrote as follows: 
“ Having consulted the works of various authorities on this 
subject, I find the views expressed in the writings of Dr. Wallich 
(particularly in his paper ‘On Triceratium, vol. vi. ‘ Quart. 
Journ. Mic. Se.’ (1858), p. 242; and ‘On the Development and 
Structure of the Diatom Valve, vol. viii. ibid. (186V), p. 129), 
most in accordance with my own independent researches. Dr. 
Wallich appears to have been the first to set forth clearly that the 
middle piece or ‘zone’ consists, while the frustule is intact, of two 
distinct plates, the one received within the other; and that the 
growth of such plates can only take place at the free margins, or 
those which are not connected with the valves; and he has also 
shown how the capacity of the frustule may be augmented, at least 
in one direction, by the sliding out of the plates or ‘rings,’ 
telescope-fashion, to accommodate themselves to the increase of the 
contents during division. . . . Dr. Wallich, I think very success- 
fully. refutes the idea of a continuous growth of the diatomaceous 
frustule, the fact being, as he states, that ‘ variation in the size of 
the valve and the number of its striz’ (in any fractional part of 
an inch) ‘ proceed pari passu during the progress of division, but 
not subsequently.’ He admits that ‘growth may take place to the 
extent of new siliceous matter being secreted along the margins of 
the valve, its depth being thereby augmented ;’ but he considers it 
highly probable ‘ that the connecting zone, by which the young valve 
is protected during its secretion and consolidation, determines the 
limits of the dimensions to be attained by it.’ He states, moreover, 
that ‘in truth no two valves of the same frustule can be of the 
same size, for the new valves, being formed within the connecting 
zones of the parent frustule, must be smaller than these.’ This, I 
should think, is the essential cause of the great diversity of size 
observable in the frustules of the same species being constant and 
universal. ... It stands to reason that as the two new half- 
frustules are developed endogenously, or within their parents, the 
former must be smaller than the latter by the whole thickness of 
the siliceous investment; and this will continue to be the case 
