Mourn cro | Histology of Minute Blood-vessels. 2138 
them, the forms thus ascertained correspond precisely with those 
mapped out by the silver solution, and when, after the action of 
silver, carmine staining is resorted to, the nuclei thus made visible 
correspond in position to the places they ought to occupy, if in fact 
the silver had mapped out the cell-boundaries, as I certainly believe 
it does. Whether the discoloration is in the cell-wall, or in the 
cement or matrix by which the adjacent cells are held together, is 
a more difficult question, and one into which I do not propose to 
enter at the present time. It is enough for the 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 
interpretation 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 in veins 
of moderate size. I have found them largest and most numerous 
in veins sth of an inch in diameter or even larger, and they 
become smaller and rarer in smaller branches. They are com- 
paratively infrequent in the capillaries and still more so in the 
small arteries; the Museum however possesses preparations showing 
them in both. I have moreover concluded from my own obser- 
vations that in number and size they vary in vessels of the same 
dimensions 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 like dimensions in the urinary 
bladder of the same animal. 
In figure they are rounded, oval, or oblong. I have measured 
them as large as zo/ooth of an inch in diameter, but smaller ones 
soo00th to goooth of an inch are more common, and the smallest 
and most frequent do not exceed yotooth of an inch. Sometimes 
they present clear centres sharply mapped 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 composition of the fluid by which the opening is occupied, 
which sometimes precipitates the silver solution while at other times it 
does not, 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 I have observed 
them apparently in the cells themselves, are probably to be explained 
by the adjacent margins having from some cause escaped the in- 
fluence 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. 
X. Sketch of a photograph representing a silver staining of the 
Q 2 
