1905.] NATURAL SCIENCES OF PHILADELPHIA. 151 



with the cytoplasm of the retinula and run to the retinular ganglion, 

 where they surround the nuclei of the ganglionic cells. There is no 

 indication of long nerve processes from the ganglion cells toward the 

 eye. 



The nervous elements of the retinular cell proper consist of a differ- 

 entiated portion of the cytoplasm inside the clear area which lies out- 

 side the rhabdome. This nerve fibre can be seen best in sections stained 

 in iron hsematoxylin, where it stains black. From this fibre, which 

 starts at the distal end of the cell and runs parallel to the rhabdome, 

 smaller fibrils are given off which run into the rhabdome where they all 

 end. More properly speaking, these fibrils are further differentiations 

 of cytoplasm which lies between the main fibril and the centre of the 

 retinula. These fibrils extend from the fibre to the rhabdome along 

 the whole length of the retinula proper, so that the nerve-endings are 

 very numerous. Below the basement membrane these main fibres can 

 be traced as dark lines in the centre of the protoplasmic processes to 

 the retinular ganglion. All of these fibres are best seen on cross- 

 sections where they stand out as black dots, but they can also be seen 

 on longitudinal sections. It is probable that the cause of the black 

 color of the rhabdome in sections stained with iron haematoxylin is the 

 presence of these numerous nerve fibrils. Concerning the distribution 

 of these fibrils inside the rhabdome, I am unable to say anything defi- 

 nite, but they probably extend almost straight to the centre. In 

 material fixed in Kleinenberg's picro-sulphuric fixing fluid the rhab- 

 dome sometimes appears as a tube, and this may indicate that these 

 fibrils do not run all the way to the centre. While the innervation of 

 the ommatidium is under discussion, it might be stated that there is no 

 indication of nervous connection with any of the other cells peripheral 

 to the retinula. 



The significance of the single retinular nucleus which lies at a lower 

 level than the others of each ommatidium, is somewhat hard to explain. 

 Hickson held that some of the retinular cells of Musca had more than 

 one nucleus. In this form there are three layers of nuclei in place of 

 two, as in Apis, Hesse, on the contrary, homologizes these lower 

 nuclei, only one of which is present in Apis, with the proximal retinular 

 cells of the apterygote insect eyes. In these forms the retinula is 

 divided into two parts, one distal to the other, each of which acts alone 

 in the formation of rhabdome structure, and both have nerve fibre 

 connections with the optic ganglia. A similar condition is found in 

 some pterygote insect ommatidia. Of these two views the one of 



