

I III. I-.VK IN NORMAL AND EYELESS DROSOPHILA. 245 



ganglion, called in this paper the outer (periopticon of Hickson) 



has in general the shape of a cone of rather low altitude with the 



apex directed inward-. The surface of the eye is co\ered with 



small facets beturen which project hair like bristle-. (Plate I., 



i , and Plate III., Fig. 2.) Each facet is the outer portion of a 



-iirje ommatidiimi. In the center of the eve the oinniatidia are 



at right angle- to the basement membrane but at the -idc- of the 



oin the membrane at a considerable angle. Beneath 



the layer of facets is a layer of pseudocom - nr lens-like struct tire-. 



In the ] >n -| iar.it ion- there is often an artificial separation betu 



the facets and the pseudocone layer. Immediately under the 



idocone- there is a layer of retinuke and rhabdomerc-. The 



latter are darkly stained rods which stand out clearly in our 



I nder this layer is the basement membrane \\hirh 



separates th \e from the outer ganglion. 



A tudy of the eye under a higher magnification reveals 



more clearly the nature of each ommatidium. l.ach |iseudocoiie 

 apprar- .1- a hollow space on whose proximal side are t\\c> of the 

 lour (-..in- i cll> which |)roduce it, with their contained inn lei 

 (Semper's Plate I. l-'ig. 2). The pseudot \n- i- < haractcri-tic 

 I tlies \\ith -hort antfnn.i', and is in contra.-t \\ith the cuci.nc in 

 which the inn lei are distal to the lens. Hcncaih each p-cudm >ne 

 i- a spindle-shaped group of rods or rhabdomeres. ' >utside each 

 rhabd< urn-re is an elongated cell or retinula. Outline- bet \\een 

 the retinul.e do not show clearly in most of our l ngiiudinal 

 tions, but between retinuhf of adjacent oinniatidia, pigment 

 cells call be seen. At the ends of the -pindle the rliabdoin. 

 are close t< 'Aether, while at the middle of it tin farther apart . 



In each ommatidium there are seven retinul.e and -c\en rhabdo- 

 men ^i\ of the latter are approximately equal in -i/e. but the 

 si-\eiith i- MiialK i and seems to be joined ed:o-ui-e '" a -mall 

 lamella. Plate I ., Rg. 3 shows a cross section ( ,f .m ommatidium 

 near the di-ial end. Here the seven rhabdoinciv- are close 

 ti'^ether, and the outlines of the seven retinula cell-, -Mine of 

 \\ hit h -In >\v nuclei, can easily be made out . Plate I ., l-'i^. 4, -li< >\\ - 

 across -ection of the ommatidium proximal to that of li^.;-,. 

 Here the rh.ibdomere- are farther apart and the spindle-shaped 

 ommatidium ha- \\idened. The retinul.e are much -mailer in 

 -ection and appear a- a proiopla-mic circle around the rhaMo- 



