1905.] NATURAL SCIENCES OF PHILADELPHIA. 125 



The plan followed in this paper is to give, first, a brief description 

 of the adult eye, so that further discussion will be more intelligible, 

 and then to take up the development of the entire eye and omma- 

 tidium, followed by a detailed description of the adult conditions, 

 since that was the plan followed during investigation, and is, perhaps, 

 the order which will be most clear to the reader. 



This work was taken up at the suggestion of Dr. Thos. H. Mont- 

 gomery, Jr., now Professor of Zoology in the University of Texas, 

 and was completed under the supervision of Professor E. G. Conklin. 

 To both I am indebted for many valuable suggestions and for help 

 throughout the work. 



II. METHODS. 



Larvae and pupae were fixed in Flemming's fluid, Hermann's fluid, 

 picro-sulphuric, picro-acetic and picric acid saturated in 50 per cent, 

 alcohol, but of these the Flemming and Hermann preparations yielded 

 the best results. For the smaller larva? it was not necessary to dissect 

 before fixation, but for older larvae and pupae the head was removed 

 to make penetration easier. For adult material, where penetration 

 is difficult, the best fixative was acetic acid, generally a 10 per cent, 

 or 20 per cent, acetic solution in 80 per cent, to 100 per cent, alcohol. 

 Kleinenberg's picro-sulphuric and picric acid in 50 per cent, alcohol 

 were also used with fair results when the head was cut in two. 



The material was all cut in paraffine, and it was found that for adult 

 material long embedding was necessary, four to eight hours, to get the 

 paraffine all through the tissues. Some material was embedded for a 

 shorter time to see whether the heat had produced any artifacts in the 

 other material which was embedded for the longer period, but in such 

 cases the lens invariably separated from the retinular layer ; no differ- 

 ence was observed in the internal tissues due to long heating. 



In staining, the best results were obtained in the use of Heidenhain's 

 iron hsematoxylin, with the use of a strong mordant for a long time. 

 For material of this kind there seems to be no better stain. It was 

 found that by destaining to different degrees the various parts of the 

 eye would show differences in color, the rhabdome, for example, stain- 

 ing an intense black in rather deeply stained material. The nerve 

 fibrils of the retinula cells also stained black with this stain. Other 

 stains, such as Delafield's haematoxylin and eosine or Bordeaux red, 

 were employed with very good results. 



For depigmenting Grenadier's solution with a somewhat greater 

 per cent, of acid was used. Parker's solution was also used, though 

 the former gave the better results. 



