158 THE EYE TN EVOLUTION 



central refractile rod, the rhabdome. The rhabdome is a product of 

 the collective secretion of the cells of the retinule and has a light - 

 conducting function ; presumably in its substance photochemical 

 changes occur, the products of which stimulate the neighbouring 

 retinular cells, but the nature of the absorbing pigments has not yet 

 been elucidated. The entire group of ommatidia, each individual of 

 which is separated in some degree from its neighbour by a mantle of 

 pigment cells, constitutes the compound eye, the surface being made 

 up of the corneal facets fitting into each other to form a mosaic (hence 

 the common name " faceted "' eye), and the retinules together forming 

 the retina. The structure would therefore suggest that Hght striking a 

 retinule stimulates it as a whole and produces a single sensation, and 

 consequently the great advance in the development of the compound 

 eye is the coordination of individual elements in a unity of function. 

 The mosaic of vision is made up of the images from the individual 

 ommatidia of which there may be few or many, each of which acts in 

 the same way as a single retinal cell of the simple eye. As in the 

 ocellus, the entire structure is derived from the surface ectoderm. 



The sensory mechanism of the compound eye is not at all clear for on this 

 subject much research yet remains to be done. Most authorities accept that 

 the retinular cells are the photosensitive elements ^; these form a characteristic 

 complex for any given species and are precisely arranged, usually 7 or 8 in number 

 but varying from 4 to 20 in different species of Arthropods. It used to be 

 generally accepted that each retinular cell was a primary neurone, and certainly 

 each extends proximally as an axon which terminates synaptically in optic 

 ganglia or nuclei ; but the interesting thing is that on the few occasions in which 

 the matter has been experimentally explored, no conducted action potentials 

 have ever been demonstrated in these cells or their axons (Bernhard, 1942 ; 

 Antrum and Gallwitz, 1951). In the king-crab, Limulus, it has long been known 

 that only one active fibre can be detected in the whole bundle of axons emerging 

 proximally from the retinule (Hartline and Graham, 1932 ; Hartline et al., 

 1952-53), and Waterman and Wiersma (1954) have brought forward significant 

 evidence that this activity is associated with a characteristic eccentric cell one 

 of which is found in each ommatidivnn. In Crustaceans little work has been 

 done germane to this problem, but it would seem that the electronic spread of 

 retinal potential travels towards the first optic ganglion without giving rise to 

 any spikes (Hanaoka, 1950). In these and in Insects the conducting neurones 

 may be located in the first optic ganglion (the lamina ganglionaris) ^ which lies 

 immediately under the basement membrane of the retina. In Insects there are 

 also units comparable to the eccentric cells of Limulus, the axons of which do 

 not terminate with those from the retinular cells in the first optic ganglion but 

 in the next more proximal ganglion (Cajal and Sanchez, 1915 ; Hanstrom, 1927). 

 The evidence available to-day would, indeed, suggest the somewhat surprising 

 deduction that although the photosensitive region is near the rhabdomes of the 



• Aci' ling to Berger and Courrier (1952) the photoreceptors in the eyes of Insects 



are situate t the bases of the rhabdomes and are not represented by the longitudinal 



cells usuaL signated as " sensory ". 



" p. 5l 



