194 Macieay Menmoriat Vouume. 
An idea of the shape and relations of these elements may best be obtained by 
reterrmg to the diagram, fig. 16, Pl. xxvi, which represents the side view of these 
truncated cones. In fig. 15 is shown a semi-diagrammatic drawing representing the 
various elements of the core unnaturally separated from each other: the central 
portion shows the elements of the core in optical section. These diagrams have been 
largely constructed from teased specimens of sections of the rods. Fig. 17, Pl. xxv1., 
shows two of the elements of the core and one cell from the neighbouring layer, 
drawn to nature from such a teased specimen. The section has gone vertically 
through the core, so as to divide the cones into two nearly equal halves, which 
halves are seen as it were from the inside. Each half possesses a nucleus. 
In the natural condition these conical elements are placed one upon another as 
closely as possible, so that in a longitudinal section of the entire organ the periphery 
of the core appears obliquely striated at about an angle of 45° to the axis of the 
organ (fig. 5, Pl. xx1v., or fig. 22, Pl. xxvz.). 
The apical end of the truncated cone is formed by a sharp inflection of the upper 
edges of the epithelial lamina, which meet or nearly meet in the axis of the cone. 
In section the profiles of these inflected edges present a clubbed appearance. (This 
description and the illustrative diagrams might suggest that only a minute axial 
canal is left in the centre of the core; but such a conclusion would certainly be 
erroneous, since, as we shall by and by see, a whole leash of fibrils occupies this 
area. We believe that at their central ends the epithelial cell elements of the conical 
laminz are partially detached from one another to admit the passage of the fibrils 
referred to.) 
The intermediate layer consists of narrow imbricated nucleated cells arranged 
not unlike the tiles of a roof, at a more oblique angle to the axis than those of the 
laminee forming the core, and completely encircling this latter. These cells form the 
second nucleated circle seen in fig. 6, Pl. xxiv. External to these is a layer of 
polyhedral nucleated cells arranged in a circular manner around the last-named layer, 
and constituting the outermost layer of the shaft. The longer axes of these poly- 
hedral cells have a nearly horizontal direction. 
Surrounding the so-constituted shaft of the rod-organ one finds in the superficial 
half the general epidermis, the neighbouring cells of which are pushed from their 
usually horizontal direction into an oblique or nearly vertical position, presumably by 
the upward growth of the organ. In the deeper half it is surrounded by the dermis, 
which forms for it a compact fibrous sheath. The external relations of the organ 
will in its different parts be at once seen on looking at figs. 3, 4, and 6, Pl. xxtv., and 
info se al EAL oe. 45 
