44 THE UNIVERSITY OF OKLAHOMA 



wild flies were first sectioned and then compared with tliose of the 

 mutation. 



The structure of the normal wild fly is as follows: the com- 

 pound eye, as in all insects, is composed of a cone shaped group 

 of single eyes or ommatidia. Each ommatidium, which registers a 

 portion of the field of vision, has an outer corneal facet, a lens like 

 structure, a group of retinulae or nerve end cells with a supporting 

 axis group of rods comprising the rhabdome. Nerve fibrils pass 

 from the retinulae through a basement membrane to the distal 

 ganglion of the optic tract. This group of ommatidia making up 

 "the compound eye" is connected to the brain by a series of three 

 optic ganglia. The distal ganglion next to the eye is called by 

 Hickson the periopticon and is composed of bundles of nerve 

 fibrils. The middle ganglion, he calls the epioptican and the proxi- 

 mal, the optican. Each of these latter two consists of a matrix 

 through which is scattered nerve fibrils, the whole designated by 

 Hickson as a neurospongium. Between the optican and epiopticon 

 and between the epioptican and perioptican the nerve fibrils decus- 

 sate. The optican is connected immediately to the brain and there 

 is no true optic nerve as in crustacea. 



There has been considerable discussion in the past as to the 

 homology of these parts with the optic tract of other animals. 

 Herger regarded the slight constriction between the opticon and the 

 brain as equivalent to the optic nerve. Hickson agrees with him, 

 and considers all the structures between the lens and the brain as 

 equal to the retina, and the retinulae and rhabdome as equivalent 

 to the rod and cone layer of other forms. Wheeler, on the other- 

 liand, regards the optic nerve as peripheral to the optic ganglion. 

 For our purpose it is unnecessary to consider these homologies since 

 we are concerned only with a comparison between the normal and 

 the eyeless. 



In eye'.ess flies, as previoush' stated, there may be no ommati- 

 dia at all, a few ommatidia, or simply a number of ommatidia smaller 

 than the normal. In all cases where there is simply a reduction 

 in the number of ommatidia, all of the three ganglia of the optic 

 tract are present. The reduction in number of ommatidia is, how- 

 ever, accompanied by a reduction in the size of the optic ganglia. 



When no ommatidia are present and the fly is totally eyeless 

 there is never any periopticon. In these cases the two proximal 

 ganglia are much concentrated, so that their structure is some- 

 times difficult to make out. All of our evidence indicates that 

 these two inner ganglia still persist even though there may be no 

 external indication of an eye. 



