538 THE SENSES AND SENSORY ORGANS. 
insect is far the largest nerve in its body, that it is connected 
with enormous ganglia, and that the whole of the nerve fibres 
they imagine they have seen would not make up one-thousandth 
part of its bulk. 
No doubt the optic nerve appears much less bulky in 
specimens made by imbedding the object in paraffin; when, 
however, the optic nerve has been treated with alcohol, chloro- 
form, often with ether, and soaked for hours in hot paraffin, it 
is no longer opaque, but transparent, and all but its proteid 
constituents have been dissolved. The optic nerve seen in the 
recent condition is large and opaque, is quite white, and its 
fibres are by no means fine. Whilst the recent dioptron is 
transparent the whole of the nervous structures are opaque 
with the exception of the outer ends of the retinal rods. 
The Trachex of the Retina—Two sets of tracheal vessels may 
be said to belong to the retina. One set, which are very fine 
and which penetrate between the fibres of the optic nerve, 
form a delicate network all over each fasciculus of retinal end 
organs, as far as the outer extremity of their inner segments. 
The other, which consists of moderate-sized vessels, form a 
reticulum in the basilar membrane between the retinule and 
give off the tracheal vessels of the dioptron, which perforate 
the cuticular layer of the basilar membrane and course out- 
wards between the great rods. 
In Eristalis the vessels of this reticulum are very much 
larger than in the Blow-fly, and the corresponding vessels of the 
dioptron are also very large. Exner speaks of such trachee as 
a tapetum. 
6. THE DEVELOPMENT OF THE COMPOUND EYE. 
a. In Arthropoda generally. 
It has long been known that the number of ommatea in the 
compound eye increases with each ecdysis in many Crustacea, 
new ommatea appearing around the circumference of the eye. 
The same kind of increase takes place in the ametabolic 
