588 
MR. B. T. LOWNE ON THE MODIFICATIONS OF 
rod-like chitinous structure between them, which gradually takes their place. I am 
entirely inclined to the former view, and regard the segments of this organ as modified 
cells. 
The inner extremity of the tetraphore rests on the outer extremity of the rhabclion, 
which is swollen into an ovoid enlargement («). The highly refractive continuation of 
the tetraphore is plainly seen to be continued as a thread-like process in the axis of 
the rhabdion (a). In transverse sections this is easily seen to be composed of four 
separate fibres. The thread-like axis of the rhabdion is enlarged into two fusiform 
swellings at the outer extremity of the organ. 
The outer extremity of the rhabdion is surrounded by a number of pigment cells ; 
these send fine moniliform pigment threads over it. The pigment is of an olive-brown 
colour. The rhabdia are cylindrical and straight. 
I have at present been unsuccessful in the investigation of the ganglionic retina in 
this insect. 
VII. On the Structure of the Compound Eye in Vanessa atalanta. (Figs. 28 to 34.) 
The eyes of this insect are similar to the last-described form, but present important 
differences in the presence of lenticular facets to the cornea, in the structure of the 
tetraphore, and in the presence of a distinct facellus upon which the rhabdion rests. 
The corneal facets are strongly convex on their outer, and slightly concave on their 
inner, surface ; they are ywooTh of an inch in diameter. 
The tetrasome consists of four nucleated cells in the immature, and of four highly 
refractive spheres ( t ) containing vacuoles in the mature insect. It is placed imme- 
diately beneath the cornea. The tetraphore consists of an outer very transparent 
globe ( t enclosing an ovoid highly refractive body (t) containing vacuoles. An 
exceedingly fine prolongation of this body connects it with the rhabdion, and the 
whole floats in the fluid of the chamber (figs. 28 and 30). The chamber is prismatic, 
as in the last form ; its pigment cells are arranged in two sets : eight surround the 
edge of the corneal facet, and a second set is situated at the inner extremity of the 
chamber. Numerous fine moniliform pigmented processes are given off from these 
cells, those from the outer set interdigitating with those of the inner, and so forming 
the pigmented lining of the chamber, as in the eye of Agrion. 
The rhabdia are quadrangular in section, and are of smaller diameter at them outer 
than at them inner extremities. In transverse section some of these rhabdia appear 
to consist of five cylinders, but in the majority four of these are fused into a single 
investing sheath, enclosing an axial structure (fig. 31). Each of the external portions 
has a pigmented thread, which is easily separated from the rhabdion ; it is connected 
with a pigmented nucleus at its inner extremity. 
At the inner extremity of each rhabdion (figs. 32 and 33, a") there is a cylindrical 
cavity (a") formed by a membranous sheath from the basal membrane ; the walls of 
