524 THE SENSES AND SENSORY ORGANS. 
Lankester and Bourne must be deceptive. In many of the 
figures given by the last-named authors the ‘yellow bodies’ 
in the retina do not appear to be normal structures, and from 
what I have myself seen in many preparations bleached with 
nitrous acid are, I believe, artificial products resulting from 
the action of the acid upon the proteid of the circulating fluid. 
Nothing can be well less like a normal structure than the 
irregular masses of structureless material shown in their figures. 
I have little doubt as to the similarity of the pre-retinal 
lamellz of simple eyes and the basilar membranes of compound 
eyes, and I am quite convinced that in both the simple and 
compound eyes all the parts on the inner side of these mem- 
branes are developed as outgrowths from the central nervous 
system. 
On the Structure of the Great Rods.—Grenacher’s figures in- 
dicate that the great rods vary extremely in structure in a 
most puzzling manner in different Arthropods, and anyone 
who has worked at transverse sections of these organs knows 
how variable the appearances are which they exhibit, even in 
the same species, in different sections. Compare, for example, 
the figures given by Grenacher [222, Figs. 125,126] with those 
by Lankester and Bourne [229, Figs. 20-26] of the so-called 
retinule of Limulus. These are typical representations of 
gigantic rhabdomes. I have no reason to distrust the figures 
given by either, because I have frequently observed equal 
discrepancies in transverse sections of rhabdomes prepared 
by slightly different methods from insects of the same 
species. 
In 1882, on examining the eye of a Plume Moth, Pterophorus, 
after dissociating the elements of the dioptron with needles 
in normal saline solution, I was surprised to find that the 
great rods presented appearances which had not been pre- 
viously observed. Each great rod consisted of an ovoid 
transparent body (Pl. XXXVIII., Fig. 3 vi); this rapidly un- 
derwent changes of form evidently due to the escape of fluid 
from its interior, and ultimately split up into a number of 
empty tubules. These were neither more nor less than the 
