304 Mr. F. A. Bather on Shell-groivth 



We have next to consider how far the intussusception 

 hypothesis holds good for the sepion. I have already noted 

 some opposed facts ; but, setting aside for a time arguments 

 derived from a cross-examination of the shell itself, I proceed 

 to call witnesses of a different character. Riefstahl seems not 

 to have investigated the epithelium that lines the shell-sac ; 

 to this I paid special attention — indeed one can hardly 

 imagine a man dismissing the secretion-hypothesis without 

 examining the structure which, above all others, was calcu- 

 lated to yield valid external evidence. This mistake was not 

 made by Appellof ; he has studied both soft and hard parts 

 together, and his description, so far as I can follow it without 

 figures, seems to agree with my sections and drawings, while 

 in our explanation of the appearance we substantially agree. 

 Dr. Appellof promises a more detailed account of his investi- 

 gations. For the present purpose a short description will 

 suffice. The whole shell-sac is lined by a simple columnar 

 epithelium of variable character. Over those parts of the 

 shell that have long been formed the epithelial cells are flat- 

 tened out so as to almost form a pavement-epithelium. 

 Towards those parts of the shell in course of formation the 

 cells become more columnar. There is a distinction between 

 the cells overlying the dorsal plate and those overlying the 

 inner plate and the last septum. The former line the mantle, 

 by which all, including liiefstahl, allow that the membranes 

 of the dorsal plate [=sheatli or guard] are deposited. Their 

 appearance is shown in fig. 3. The cells that line the ante- 

 rior part of the shell-sac on the ventral side («'. e. over the 

 visceral hump) are very peculiar. I have been unable to 

 demonstrate a cell-wall ; the nuclei stain clearly with borax- 

 carmine, and the chromatin element in each is much convo- 

 luted, producing the appearance of several nucleoli. Towards 

 the sutural margin of the anterior septum the nuclei become 

 elongate, and seem to tail off by their proximal or ventral 

 ends into the underlying connective tissue {tig. 4). The 

 appearance is in fact somewhat that of a syncytium formed 

 by cells migrated from the connective tissue ; the nuclei form 

 a single layer, and have their long axes parallel to one another, 

 but slightly inclined to the surface in antero-dorsal direction. 

 Above, i. e. dorsal to, the nuclei is a clear cytoplasm, and 

 above this again a distinct layer continuous along the surface, 

 and separated in places from the cytoplasm by a dark line. 

 Over the region where the septum joins the inner plate the 

 cells are extraordinarily elongate and their nuclei correspond- 

 ingly dragged out (fig. 5). The cells show a greater degree 

 of separation in this part, but the superficial layer is still cou- 



