334 N. E. BROWN ON THE STRUCTURE OF DIATOMS. 



when examined under dark-ground illumination the reverse 

 sterns the case, for then the plate-substance appears to have 

 the transparent of a black sky, and the membrane of the pits 

 reflects the light so as to appear like minute golden stars. It is 

 by some refractive or diffractive property of this membrane that 

 the black-dot appearance is produced, for when the membrane 

 itself is accurately in focus no black dot is seen ; but if the focal 

 plane of the lens is above the focus of the membrane, then the 

 black-dot appearance is produced, and appears to me nothing 

 more than a deceptive light effect. From the different appear- 

 ances of this membrane under different methods of illumination 

 and its contrast with that of the plate, I think it must be of a 

 somewhat different nature. Although I suspect that it is per- 

 forated, I have quite failed to perceive any trace of pores in it ; 

 higher magnification than I am able to obtain is probably needed 

 for demonstrating anything of that nature. 



In conclusion, from the evidence afforded by Mr. Capell's slide 

 and from the observations I have made upon other diatoms not 

 hastily formed opinions, but based upon many hours' examination 

 under all forms of illumination it seems clear that we can no 

 longer regard all the black dots usually seen upon diatoms as 

 being pores through the shell, although there may be cases 

 where they are so ; for in the cases examined they are certainly 

 nothing more than light effects or shadows, either caused by the 

 nodes of the grating structure, as in Pleurosigma ; or by the 

 membrane closing the meshes of the grating, as in Stauroneis ; 

 or by the membrane closing the pits in the cell-wall, as in 

 Triceratium. 



What I take to be the true pores must be sought for in the 

 thin membrane of silex closing the meshes or pits. If these 

 are not pores, then I do not know where we are to seek for them. 

 I think it must be perfectly obvious, to all who like myself have 

 carefully studied the movements of living diatoms, that there 

 must be openings or pores through the shell communicating with 

 the interior. This seems also conclusively proved in cases where 

 the shell certainly has chambers in its substance, as in Triceratium 

 favus, Pleurosigma angulatum and others, for in the ordinary 

 process of mounting the medium penetrates easily into the 

 interior of the cavities, and they can also be filled by chemical 

 deposits, which I do not think would be the case if the membranes 



