214 Histology of Minute Blood-vessels. [Mp Oct bro. 
external epithelium of the frog’s skin. Negative No. 22, New Series. 
From preparation No. 3036, Microscopical Section. Magnified 400 
diameters by Wales’s 1th. The preparation was made by myself. 
The epithelium of this surface consists of a number of layers, and 
the silver has penetrated in different portions of the skin to various 
depths. In the photograph 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 ys/yoth of an inch in diameter. 
Many of the nuclei have been somewhat tinted by the silver, a cir- 
cumstance which is not infrequent if the silver action is intense. 
In the boundaries of the epithelial cells may be seen very many 
little rings, with black margins and clear centres, averaging sp'ooth 
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 which 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 represents an 
outline of a portion of this photograph ; 
a, a, the nuclei of the epithelial cells; 
b, b, the stomata; e, e, stomata which have 
become black and opaque throughout. It 
has been urged, however, by Balogh, that 
even if the stomata described in the vas- 
cular 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 s,'5oth of an inch in diameter. But even 
if we discard the supposition that the pores may be stretched open 
and made large by the distended condition of the vessels of inflamed 
parts, there appears to me no difficulty in understanding how a 
white corpuscle might pass through the smallest of the stomata I 
have described. An opening y+5)ooth of an inch in diameter is only 
a little less than one-third the average diameter of the white cor- 
puscles, and anyone who has seen the extraordinary modifications 
of form which these little masses of protoplasm undergo in the 
course of their so-called “amoeboid movements,” would readily credit 
their capability of passing through such apertures. As the amoeboid 
movement does not occur in the white corpuscles while rolled along 
in the torrent of the circulation, but only when the movement of 
the blood is arrested more or less completely, the fact that large 
numbers of white corpuscles do not habitually pass through the 
vascular walls into the tissues will not militate against the notion 
of patulous orifices. That a passage of the white blood corpuscles 
