8 REPORT — 1844. 



striation in question. The membrane evidently projects inwards at those 

 parts, not in consequence of being pushed inwards from without, but be- 

 cause its own thickness is there increased. This appearance corresponds well 

 with the conclusion already drawn, in regard to the progressive formation of 

 each layer of shell ; and I am much inclined to believe that each transverse 

 marking indicates a distinct deposit. Whether, during the time when this 

 succession of deposits was taking place, the prismatic cells grew at their 

 bases, and these lines indicate the additions which were progressively made 

 to the length of the cells, — or whether the long prismatic cells, as we now find 

 them, are made up by the coalescence of a number of layers of flat pavement- 

 like enithelium-cells, placed one upon another, and the lines indicate their 

 points' of junction, — I do not feel warranted in affirming with certainty, as the 

 question could be only rightly decided by examining the shell in the progress 

 of its formation, wliicli I have not yet had the opportunity of doing. I am 

 much inclined, however, to adopt the latter view ; which was suggested to uie 

 by Professor Owen. The coalescence of cells, linearly arranged, so as to 

 form a single long cell or tube, is an occurrence with which Animal and Ve- 

 getable Physiologists are alike familiar. The idea derives strength from the 

 fact, that I have occasionally met with a layer of prismatic cellular structure 

 of such extreme tenuity, that it was almost impossible to separate it, lying 

 between thicker layers of the same in the shell of Pinna. The cells of this 

 layer, instead of being elongated prisms, were flat and pavement-like, resem- 

 bling the epithelium of serous membrane ; and it was in such that 1 have 

 found the cytoblasts most perfectly preserved (fig. 8). It is hardly to be 

 supposed that this layer was produced by a distinct act of shell- formation, as 

 it would not add in any appreciable degree to the size or solidity of the shell ; 

 and it seems probable that it was a supplemental portion, which had not 

 coalesced with the remainder of the layer, of which it should properly have 

 formed a part. 



14. The last point to which I shall advert, is one which I have already 

 noticed, — the presence of dark or semi-opake cells in great numbers on the 

 natural outer surface of the layers of prismatic cellular substance in the 

 Pinna (fig. 5) ; their presence in a much diminished proportion, and only as 

 small cells, in sections taken from the interior of the layer (fig. 6); and 

 their complete absence (in general at least) from thenatural internal surface 

 of the layers (fig. 7). I have nearly satisfied myself, that the appearance of 

 opacity is due to the presence of a small quantity of air in or near the ex- 

 tremities of the cells. That this, being enveloped in a substance of so high 

 a refracting power as carbonate of lime, would give the appearance of opa- 

 city, is easily understood on optical principles, and is practically well known 

 to the microscopist. Now when we consider that the exterior surface, on which 

 this appearance is chiefly seen, is the one furthest removed from that surface 

 on which the carbonate of lime is being poured forth, it does not appear 

 surprising that the calcifying substance should not always find its way to the 

 ends of the cells, but should occasionally leave a void space there. And 

 when it is remembered that the dark cells of the interior of the layer are few 

 and small, and that, as already shown, these small cells are the sections of the 

 acute terminations of prisms Avhich do not pass on to the surface, it is obvious 

 that the same view fully accounts for their occurrence in this situation. 



15. Although the prismatic cellular structure has not yet been observed 

 in actual process of formation, yet certain appearances which are occasionally 

 met with in the marginal portions of its newest layers, throw great light upon 

 its mode of growth, and indicate its strong resemblance to cartilage in this 

 respect ; for in these situations we find the cells neither in contact with each 



