5i8 THE 5/:.Y.s7:\Y A.\I> S/-:.\SORY ORGANS. 



of the compound eye is that the great rods are recipient struc- 

 tures, and that they are the terminals of the optic nerve. 

 That this view rests on hypothesis and imperfect knowledge 

 will appear hereafter, and I shall show that it has never been 

 proved, and that it is not tenable on developmental, anato- 

 mical, or physiological grounds. I shall commence by review- 

 ing the evidence on which it is asserted that the dioptron is a 

 recipient organ concerned in the transformation of light vibra- 

 tions into a nerve stimulus. 



Leydig [208] termed the great rods nervous rods, and Max 

 Schultze [210] visual rods, ' Sehstabchen ' ; and it has been 

 held by all previous writers on the subject that the basilar 

 membrane is perforated by the terminal fibres of the optic 

 nerve, which are usually supposed to end in one or other of 

 the structures of the dioptron. 



So far as the general arrangement of the parts of the great 

 eyes of Arthropods are concerned, and the arrangement of the 

 different layers, my description given above agrees well enough 

 with that of previous authors ; but the interpretation of their 

 nature and their minute structure is altogether different. The 

 first point which struck me years ago was that in some insects 

 at least the basilar membrane is absolutely imperforate, and 

 the second was that the so-called outer layers of the optic 

 ganglion have a structure which is identical with the retina of 

 the simple eye, and only differs in detail from the rod and cone 

 layer of the vertebrate retina. If these points are established, 



I'TIo.N til- J'l.Ail \\XVI. 



Fi<;. I. A section of a portion of the compound eye of the Blow-fly seen \\iih a 

 J inch objective, f, the compound cornea; c c, the crystalline cone (pscudo- 

 cone) ; ;', iris cells ; /, the corneal lens ; ;// /, mcmbrana b;isihuis ; ;/'. nuclei of 

 the iris cells ; ;/'-', nuclei of the implemented cells of the sheath of the ihabdome ; 

 //', nuclei of the inner pigment cells ; 4 , nuclei of the inner limiting membrane ; 

 <' ;/, optic nerve fibres; r/, outer segments, and ;-/', inner M.-gim-nts of the 

 retinal rods or bacilli ; rt n, retinal nuclei ; v, chaplct cells of Yiallanes. 



FH;. 2. Transverse sections of the ommatea seen with a ,' oil immer.-ion objective. 

 a, section through the pseudo-cone ; /', section through the iris cells ; f, section 

 through the rhabdomes of four ommatea. 



