BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 149 
The shaft of a push-rod consists of four concentric layers 
of cells, which are rendered quite distinct by their unequal 
staining (figs. 3 and 4). Wilson and Martin regard the 
outermost of these layers as belonging to the general epidermis, 
and forming “a kind of follicle’ round the rod proper. I 
took the same view in 1884, speaking of the third layer as 
“followed by concentrically arranged fusiform cells belonging 
to the general epidermis” (lI. c., p. 16). The innermost im- 
bricated cells, constituting the first layer, are looked upon by 
Wilson and Martin as hollow truncated cones made up of three 
or more cells. This interpretation is certainly supported by 
the teased preparation represented in their pl. xxvi, fig. 17. 
I could never make out any cell outlines in this layer when 
studied in transverse sections of the rods, although three or 
more nuclei were often seen (fig. 3). Longitudinal sections, 
on the other hand, show the outlines of successive cells very 
distinctly (fig. 4). My observations to this extent support 
Wilson’s and Martin’s conclusions as to the nature of the 
innermost cell-structures. As to the second layer of imbri- 
cated cells, and the larger far less sloping cells constituting 
the third layer, we are in entire agreement, and my fig. 4 
would, in this respect, serve for the illustration of Wilson’s 
and Martin’s description. It should be noted, however, that 
the nuclei of the cells of all the layers are surrounded by 
pigment masses and granules. 
The central group of fine filaments occupying the axis of the 
rod, together with the ring of similar structures placed between 
the first and second layers of imbricated cells, are very con- 
spicuous, and early attracted my attention (1884, 1. c., p. 16). 
They are clearly seen in figs. 3 and 4, Longitudinal sections 
show that they pursue a parallel course along the rod except 
at the convex lower extremity, where I could just make out, 
in the most favorable preparations, that the filaments of the 
central group diverge in a brush-like manner (fig. 1). The 
most careful examination, under very high powers, of the 
best material I could command, displayed a structure which 
is represented in fig. 4. Each filament appeared to be 
