244 MILDRED HOGE RICHARDS AND ESTHER Y. FURROW. 



A superficial examination of the heads of eyeless flies revealed 

 the fact that those totally eyeless have no external trace of 

 ommatidia. An interesting question, first suggested by Dr. 

 T. H. Morgan, arose concerning the condition of the optic tract 

 in the totally eyeless flies. The heads of such flies are greatly 

 reduced in size resembling pin heads. Since the eye is gone and 

 the head so greatly reduced, it would seem highly probable that a 

 large part of the optic tract may be wanting. Accordingly a 

 comparative study of the heads of normal and eyeless flies was 

 undertaken. Sections were cut and a special study made of the 

 optic tract. No attempt was made to study the brain nor was 

 the optic tract studied neurogically. This had been done for the 

 bee by Kenyon. The problem was to compare the optic tracts of 

 normal and eyeless strains. In other words what part of the 

 optic tract is missing in eyeless? 



TECHNIQUE. 



The flies were anaesthetized with ether. Then with the aid of 

 a lens the heads were cut with a razor blade from the body so as to 

 include most of the thorax as well as the head, since it was found 

 that subsequent handling was considerably facilitated by the 

 presence of the thorax. The desired portion thus cut oft was 

 immediately immersed in the killing fluid. Several killing fluids 

 were tried but the most satisfactory was Bouin. Any picric acid 

 combination proved satisfactory, and considerable success 

 followed the use of a warm saturated solution of picric acid in 50 

 per cent, alcohol. Material was allowed to remain in Bouin 24 

 hours. It was then dehydrated and embedded in paraffin. 

 Sections were cut at various thicknesses, 5, J\, 10, and 15 microns. 

 For general study the thickness of 10 microns was most practical. 

 Both ha?malum and iron harnatoxylin were used as stains. For 

 detailed work the latter was more satisfactory and was used 

 extensively throughout the study. In a few cases eosin was used 

 as a counterstain. 



DESCRIPTION. 



The eye of Drosphila is very large and occupies almost the 

 whole side of the head (Plate III., Fig. i). Our sections revealed 

 the structure of the eye. It is of the large compound type 

 characteristic of insects. The whole eye with its most distal 



