391 



teriial epitlielium of the frog's skin. Negative No. 22, New 

 Series. From preparation 3036, Microscopical Section. 

 Magnified 400 diameters by Wales's ^th. The preparation 

 was made by myself. The epithelium of this surface consists 

 of a number of layers, and the silver has penetrated in dif- 

 ferent portions of the skin to various depths. In the photo- 

 graph the epithelial cells of the upper surface are sharply 

 mapped out, while the boundaries of the cells of several of 

 the deeper layers are seen out of focus beyond. The cells 

 are hexagonal in shape, and average l-1300th of an inch in 

 diameter. Many of the nuclei have been somewhat tinted 

 by the silver, a circumstance which is not unfrequent if the 

 silver action is intense. In the boundaries of the epithelial 

 cells may be seen very many little rings, Avith black margins 

 and clear centres, averaging l-5000th of an inch in diameter, 

 and also many similar forms, of the same size and occupying 

 like positions, which are quite black and opaque throughout. 

 In some parts of the preparation, from Avhich the photograph 

 was taken, almost all the rings are black and opaque, while 

 in other portions almost all present clear centres. The view 

 which regards these rings as true pores certainly appears to 



me to require fewer suppositions than any other. The cut 

 rej^resents an outline of a portion of this photograph ; a, a, the 

 nuclei of the epithelial cells ; b, b, the stomata ; c, c, stomata 

 which have become black and opaque throughout. It has 

 been urged, however, by Balogh, that even if the stomata 

 described in the vascular epithelium are admitted as such 

 they are not large enough to permit the passage of the white 

 blood-corpuscles which, as is well known, average about 

 l-3000th of an inch in diameter. But even if Ave discard 

 the supposition that the pores may be stretched open and 

 made larger by the distended condition of the vessels of in- 

 flamed parts, there appears to me no difficulty in understand- 

 ing how a white corpuscle might pass through the smallest 

 of the stomata I have described. An opening 1-lOOOOth of 

 an inch in diameter is only a little less than one third the 

 average diameter of the white corpuscles, and any one Avho 



