THE EYE IN NORMAL AND " EYELESS " DROSOPHILA. 25! 



membrane to the eye, but that the fibers ended in the outer 

 ganglion; these conclusions seem not to have been confirmed by 

 other workers. Most modern investigators agree with Parker, 

 who ha- identified the rhabdomeres as the retinal elements, and 

 has been able e\ peri mentally to produce upright images in them. 

 His experinu-nt.il < onclusions are borne out by his detailed work 

 (Crustacea ) on the structure of the rhabdomeres as well as by that 

 of Hesse. Tin- Litter has been able in in-eot- to trace nerve 

 til.ril- pa--ing from the rhabdomeres to the retinula?, each 

 rctiniil.i ending in a nerve fiber which goes through the basement 

 membrane. 



Sim x- tin- rhabdome layer is to be regarded as tlu- true retina it 

 \\o,ild -i in that tin- optic nerve should properly be i on>idcred as 

 dir-t.il t" the ganglia rather than as between inner ganglion and 

 br.iin .n.rding to Berger's conception. Thi- is the \ iew taken 

 by Wheeler who regarded the optic mi\i in anta .1- peripheral 

 to tin- optic ganglion. The present \\ >rk \\oiild -cciii to lend 

 some -iipport to the idea that the mas- of de< n--ating libers be- 

 tween the outer and middle ganglia is the optic ncr\e. for when 

 the eye disappears, it would be most natural to < the break 



in the optic tract to take place at the optic ner\e. It mu>t be 

 jioinied out that we lack definite embryological infoMnat!"ii as 

 ,D tin- origin of the outer ganglion, and until this j> obtained, 

 the >ui;v;->tion just made can be only pro\ i>ion.d. 



SUMMARY. 

 1 lie> of the imitation eyeless (Drosophiia melanogaster) have 



sin, ill eyes on both y-ide^ or are sometimes totally e\eless on 

 either or Imth >ide>. I he eye and the optic tract in normal and 

 eyeless BtO k- were >tu<lied in order to determinate what part of 

 the optic tract i> lacking in the eyeless llie-. In the normal fly, 

 three ^airjia, outer, median and inner, connect the eye \\ith the 

 brain. In tlie> \\ith -mall eyes all three ganglia are j>ri>ent and 

 the ommaticlia aie normal although greatly reduced in number. 

 In totally eyeless tlie> the outer ganglion is mining and the 

 median and inner are contracted into a more or less shapeless 

 ma \\hich ne\ ert heUss discloses its double nature. 



Since the retina has been identified by modern workers as the 

 rhabdome layer of the eye, these observations lend support to the 



