390 



tlie purposes of this paper that the peripheries of the cells or 

 the substance just external to them, exhibits a much more 

 speedy and intense reaction with the nitrate than the cell 

 contents do, and must therefore differ more or less from these 

 in composition. 



Having arrived at this conclusion with regard to the 

 general interjDretation of the action of silver on epithelial 

 surfaces^ the question of the true meaning of the so-called 

 stomata next demands consideration. They are to be observed 

 most abundantly, as may be inferred from the photographs 

 described, in veins of moderate size. I have found them 

 largest and most numerous in veins l-50th of an inch in 

 diameter or even larger, and they become smaller and rarer 

 in smaller branches. They are comparatively infrequent in 

 the capillaries and still more so in the small arteries ; the 

 Museum, however, posseses prejiarations showing them in 

 both. I have, moreover, concluded, from my own observa- 

 tions, that in number and size they vary in vessels of the 

 same idirnensions in different parts of the body. Thus, for 

 example, in the veins of the mesentery of the frog they are 

 larger and more abundant than in veins of the lilce dimensions 

 in the urinary bladder of the same animal. 



In figure they are rounded, oval or oblong. I have measured 

 them as large as 1 -4000th of an inch in diameter, but smaller 

 ones* l*5000th to l-6000th of an inch are more common, 

 and the smallest and most frequent do not exceed 1- 10000th 

 of an' inbh. ■ Sometimes they present clear centres sharply 

 mappied out by black boundaries, sometimes forms of the 

 same size and character are opaque and black throughout, 

 and this has been interpreted as due to variations in the com- 

 2)osition of the fluid by which the opening is occupied, which 

 sometimes precipitates the silver solution while at other times 

 it does riot, and the action is limited to the solid margins of 

 the pore. They are almost invariably found in the marginal 

 line between adjacent epithelial Cells, and the rare cases in 

 which t have observed them apparently in the cells them- 

 selves, are probably to be explained by the adjacent margins 

 having from some cause escaped the influence of the silver 

 salt. From my study of these peculiar inter-cellular forms, I 

 am inclined to regard with favour the opinion that they are 

 actual openings in the epithelial layer. It may aid others in 

 arriving at a conclusion on the subject, if I here present a 

 photograph of the stomata in the external epithelium of the 

 skin of the frog through which, as is well known, a rapid 

 transiulation of liquid habitually occurs. 



X. Photograph representing a silver staining of the ex- 



