138 
POPULAR SCIENCE REVIEW. 
rigidity and impermeability of the material would be incom- 
patible with the production, by the same means, of a second 
independent siliceous layer altogether externally to the first. 
On the other hand, if we regard the valves and connecting 
zones as the product of the colourless formative protoplasm 
which is present not only within the chamber, but sends forth a 
delicate film to invest exteriorly the already silicified parts (free 
communication being, as already pointed out, afforded between 
the contents of the chamber and the surrounding water by 
means of the apertures in the margins of the valves, and also 
between the plates of the connecting zones themselves), all 
difficulty vanishes, and the observed structure of the Diatom 
frustule becomes forthwith reconcilable with the formative 
faculty which has, under any view of the case, to be assumed as 
the agency whereby the mineral secretion is effected. As it is 
very difficult to make all this clear by mere description, I have 
endeavoured to give a general idea of the structure of the 
Diatom frustule, and the relations of the valves and connecting 
zones to each other, by the subjoined diagram, fig. 3. 
Fig. 3. 
ABC 
A. Front view of a naviculoid Diatom frustule when just completed, and before 
division has begun to take place. The central double line represents the two 
valvular margins ; v v, the two valves. 
B. The same frustule when division is taking place, and the connecting zones 
c z, c z , have been formed. 
C. Front view of one of the valves of same. 
It has already been seen that in the Diatomacese, as in the 
Desmidiaceae, there exists the same division of the protoplasm 
into a colourless formative portion, and one more or less bril- 
liantly coloured which constitutes the true endochrome, and 
contains within its substance the laboratory and materials, so to 
speak, which are requisite in the processes of division and repro- 
duction. In the just completed frond of the Diatom the true 
endochrome occurs in two halves, each of which is so perfectly 
distinct in outline as to warrant the conclusion (when coupled 
with what has been noticed as being the case in the Desmid) 
that it is in like manner enclosed by an investing wall, although 
the extreme tenuity of this has heretofore prevented it from being 
distinguished. But it is well to bear in recollection that in the 
