ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 51 
true skin, and especially follows the course of the blood-vessels, entering into 
the composition of the cellular coat of the arteries and veins, and twining about 
the capillaries in a very remarkable manner. The walls of these cells and of 
their tubular offsets appear to be extremely delicate, and some attempts which 
I have made to isolate them from surrounding tissues have barely served to 
demonstrate their existence. The cells vary considerably in dimensions accord- 
ing to the size of the animal; thus, those in Figs. 8,9, and 10, which are from 
young frogs, though magnified 500 diameters, show in the drawing even 
smaller than those in Fig. 3, magnified only 250 times, the latter being from 
a full-grown specimen. In an average full-sized cell of a large frog, the middle 
portion was found to measure I-330th of aninch in length by 1-670th of an inch 
in breadth, and 1-1500th of an inch in thickness. The last-named dimension was 
obtained by carrying the focus of an object-glass of high power, from the most 
superficial to the deepest part by the screw for giving slow motion, and reading 
off on its graduated circle the number of divisions traversed, these having 
a known proportion to the depth measured. Opportunities for testing the 
correctness of this measurement were presented by other cells which lay edge- 
wise, so that their thickness could be observed directly. 
Perhaps the strongest argument in favour of the cellular nature of these 
receptacles of colouring matter is afforded by the universal presence of a nucleus 
in the central cavity of each. In large frogs it is often difficult or impossible 
to discover clear evidence of it, but in small ones, in which the web is much 
thinner and its constituent parts therefore capable of clearer definition with 
the microscope, it can be quite distinctly seen in the reticular condition of 
the pigment. Its form and relations may be gathered from Figs. 8,9, and Io. 
In Sand ro the bodies of the cells are viewed on the flat, and the nucleus appears 
as an oval colourless body, about I-2500th of an inch long by 1-3300th of an 
inch broad. In Fig. 9 the body of the cell is seen edgewise applied to the wall of 
a capillary blood-vessel, which is embraced by its processes. The thickness of 
the nucleus is thus displayed, and is shown to be equal to that of the cell in 
which it lies, which in fact it causes to bulge slightly, and also nearly as great 
as the breadth of the nucleus in Figs. 8 and ro. In the cell of Fig. 10, the 
thickness of the nucleus, measured in the manner above described, was found 
about equal to its breadth. The nucleus in Fig. 8 is not centrally placed 
in the body of the cell, and I have in some other cases seen it still more 
eccentric.? 
The contents of these cells are very minute dark granules or molecules sus- 
* The precise relations and dimensions of the nucleus have been ascertained subsequently to the 
reading of the paper. 
E2 
