20 
G. H. Parker 
elongates about Ye its leugth, i. e., it contracts on the lines of its 
fibres, which are at right angles to its length, in precisely the same 
way that the constituents of the nerve fibre do; in other words, the 
substance of a nerve fibre and of the fibres of the rhabdome behave 
similarly towards heat. For these various reasons I have been led 
to regard the fibres of the rhabdome as nervous structures, 
the distal ends of the fibrillae of the retinal nerve fibres. 
As can be seen in transverse sections of the half-plates. these 
nerve fibrillae constitute the greater part of the mass of the rhab- 
dome, the interfibrillar substance being limited to a comparatively 
small space. Whether the fibrillae are simply fluid-filled pores in 
this substance or delicate rod-like bodies imbedded in it is difficult 
to say, but whichever they may prove to be seems to me to have 
little bearing on their nervous nature so long as the same question 
concerning nerve fibrillae in general remains open. 
If the finer structure of the rhabdome, as presented in the fore- 
going Paragraph, is not directly opposed to Grexacher’s (79, pag. 158) 
conception that this body is the cuticular product of the retinular 
cells, it öfters, at least, little Support for that view and still less for 
Watase’s (90, pag. 291) opinion that the organ is a chitinous pro- 
duct. To me the rhabdome seems in no sense a secretion, but 
rather a differentiation of a portion of the protoplasm of the retinular 
cell, much as muscle substance is the product of a muscle cell, a 
view already expressed by Johansen (92, pag. 353) in bis prelimi- 
nary account of the development of the eyes in Vanessa. 
The diagrammatic figure 60 (PI. 3) gives, by way of summary, 
the complete outline of one of the seven elements that unite to 
form a retinula. The swollen distal end containing a nucleus tapers 
into a neck, which forms part of the wall of the pear-shaped cavity 
and which leads directly to the body of the element. The axial 
side of the body is marked by about ten transverse waves, each of 
which carries on its crest a Segment of the toothed rhabdomere. The 
proximal end of the body becomes fibrous, pierces the basement 
membrane, and, as a retinal nerve fibre, extends to the first optic 
ganglion where it ends in many fine branches. The fibrillae from 
the Segments of the rhabdome unite in the body of the element to 
form its fibrillär axis, which is continued proximally as the axis 
cylinder of the retinal nerve fibre and resolves itself into fine bran- 
ches in the first optic ganglion. In all parts of the element, except in 
the true nervous structures just mentioned and in the nucleus, there 
