Wallich^ on the Diatom-valve. 
133 
modify this view by supposing that an addition may take 
place to the actual number of the striae. 
Now^ whilst I fully admit the correctness of the first of 
these premises^ — namely, that whilst the total number of 
striae on a valve is nearly uniform in all the individuals of a 
given species orvariety, the number of striae upon the fractional 
part of the valve admits of very great variation, — I dissent 
entirely from the opinion that the number of the striae in a 
fractional part of a valve is capable of modification, either 
by the extension of the valve through growth, the total 
number of the striae remaining the same, or by additional 
striae being produced within the limits already attained. 
The fact is, that the variation in the size of the valve and 
the number of its striae proceeds, pari passu, during the 
progress of division, but not subsequently. Growth may 
take place to the extent of fresh siliceous matter being 
secreted along the margins of the valve, its depth being 
thereby somewhat augmented ; but it is highly probable, for 
reasons which shall immediately be adduced, that the con- 
necting zone, by which the young valve is protected during 
its secretion and consolidation, determines the limit of the 
dimensions to be attained by it ; and although the young 
valve may still have to undergo a certain degree of conso- 
lidation, the whole of the characters, as we observe them 
under the microscope, are indelibly and unalterably impressed 
upon it, either before or almost immediately after its libe- 
ration. In like manner, the two rings of the connecting 
zone grow lengthwise by secretion of fresh siliceous matter at 
one of their margins only — as was shown by me in a former 
communication to the ^Microscopical JournaP (vol. vi, p. 
224) — and they are thus enabled to slide out, one from the 
other, telescope fashion, and to accommodate themselves to 
the increase of their contents during division. The last 
feature is strikingly manifest in such genera as Biddulphia, 
Amphitetras, Isthmia, Grammatophora, and others. 
1 believe I am quite correct in stating that it very rarely, 
if ever, happens that an imperfectly developed — that is, an 
immature — valve is found associated with one of the parent 
valves from which it was derived, after the separation of the 
parent connecting zone; whereas we constantly meet with 
such a combination prior to that event. In the next place, 
whensoever we find, through the evidence of the still per- 
sistent connecting zone, that a young valve has but recently 
been perfected, its structure presents no peculiarity whereby 
it can be pronounced to differ from the parent valve with 
which it is associated. We frequently meet with frustules, 
furnishing incontestable evidence of recent division having 
