518 THE SENSES AND SENSORY 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, ‘Sehstibchen’; 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, 
DEscRIPTION OF PLATE XXXVI. 
Fic. 1.—A section of a portion of the compound eye of the Blow-fly seen with a 
inch objective. c, the compound cornea; ¢ c, the crystalline cone (pseudo- 
cone) ; 2, iris cells; /, the corneal lens ; » 4, membrana basilaris ; 7!, nuclei of 
the iris cells ; 7°, nuclei of the unpigmented cells of the sheath of the rhabdome ; 
n*, nuclei of the inner pigment cells ; 74, nuclei of the inner limiting membrane ; 
0 ”, optic nerve fibres; 7, outer segments, and 72}, inner segments of the 
retinal rods or bacilli; 7¢ », retinal nuclei ; v, chaplet cells of Viallanes. 
Fic, 2.—Transverse sections of the ommatea seen with a y, oil immersion objective. 
a, section through the pseudo-cone ; 4, section through the iris cells; c, section 
through the rhabdomes of four ommatea, 
