THE FROG 31 
preserved specimens are kept. Be sure that you have obtained a 
portion only one cell in thickness; this must be mounted in water, 
and covered with a cover slip. The cells of this layer are much 
flattened and, as a rule, polygonal in outline; each of the poly- 
gonal cells contains a nucleus near its center. Among the ordi- 
nary epithelial cells of the stratum corneum may be found occa- 
sional smaller cells with rounded outline and a central tri-radiate 
opening; these are the stoma cells, through which the mucus 
secreted by the glands of the skin escapes to the exterior. Sketch 
a few cells of the stratum corneum, including a stoma cell. 
B. With a hand lens study the skin of the frog, observing the 
differently colored pigments and the small papillae, formed by 
groups of poison glands and mucus glands, found most abun- 
dantly on the dorsal surface. In the lighter areas the black pig- 
ment may be seen to be grouped in small irregular spots formed 
by the pigment cells. 
C. Cut out a piece of the integument containing a moderate 
amount of pigment, scrape its inner surface in order to render it 
thinner, mount it on a slide with a little water and cover it with 
a cover slip. Observe, on focusing upon the outer surface, the 
cells of the horny layer with here and there small round trans- 
parent areas marking the location of the minute stoma cells 
with their tri-radiate openings. Underneath the stoma cells lie 
mucus glands whose ducts lead to the surface at the tri-radiate 
openings. In one of the deeper layers of the skin, the cutis, 
numerous black pigment cells will be found; some of these may 
be much extended and their processes profusely branched, while 
others appear to be in a much contracted condition. These dif- 
ferences are best seen by comparing the skin of a dark frog with 
that of a light one. The changes in these pigment cells cause the 
color changes in the skin. Sketch a black pigment cell showing 
numerous branches, and another in the contracted condition. 
Study the skin both by reflected and by transmitted light; do you 
find any other kind of pigment cells besides black ones? 
D. Study a cross-section of the skin. The skin is divided into 
two principal layers, the outer or epidermis and the inner or 
cutis. 
1. In the epidermis, which consists of several layers of cells, 
