THE COMPOUND EYE OF THE BLOW-FLY. 537 
In Pl. XXXVIII., Fig. 5, some of the various forms assumed 
by these elements are represented, the most remarkable of 
which are perhaps the twin cones. I am unable to localise 
the precise localities characterised by these, as I have only 
seen them in specimens in which the retinal elements were 
dissociated by teasing with needles. 
The Optic Nerve, the external chiasma of Viallanes, consists 
of large, very definite fibres crossing each other in the hori- 
zontal plane. In preparations fixed with osmium peroxide 
these fibres are much blackened and their outline is very 
distinct. In preparations made after prolonged treatment 
with alcohol and chloroform, as in all paraffin imbedded 
sections, the optic chiasma has lost its opaque character. In 
many insects, as in Noctuids, the optic nerve fibres form huge 
bundles which suddenly cease in the end organs of my retina. 
The fine fibres supposed by Hickson, Patten, and others to 
enter the great rods, are minute structures. Their aggregate 
bulk is not, perhaps, one thousandth part of that of the fibres 
of the optic nerve. If the great bundles of nerve fibres of the 
chiasma passed into the great rods the connection would be 
apparent enough. But no one has seen such a connection. 
It is true Hickson gives a figure [237, Fig. 21] of what he 
takes to be the passage of nerve fibres into the dioptron in 
Agrion bifurcatum, but his nerve fibres end abruptly, and the 
supposed connection consists of cells which, I have not the 
slightest doubt, are in reality connective-tissue elements. I 
cannot understand how he reconciles this figure with such 
figures as he gives of what he terms his neurospongial net- 
work [287, Figs. 16, 17, 31-32]. 
The supposed terminals in the great rods and cone described 
by Patten, and in the great rods by Hickson, are as fine as the 
intra-epithelial corneal nervous network of a vertebrate, and 
bear about the same proportion to the optic nerve of the 
Arthropod as this bears to the optic nerve in the Vertebrata. 
The authors who maintain that such fine fibres exist in the 
dioptron and are really nerve terminals of the optic nerve, 
seem to have lost sight of the fact that the optic nerve of an 
