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 [237, 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 



