224 ME. J. W. HTLKE ON THE CHAMELEON'S RETINA. 



an oval or stellate vacuole, regarded by Muller as a nucleus, and which is not to be 

 mistaken for an outer granule. 



The difference between these extreme forms of cones in the centre and in the peri- 

 phery of the retina is so great, that, judging from their shape alone, they might not be 

 unreasonably regarded as different elements ; but a regularly progressive change from 

 one to the other shape in passing from the centre of the retina outwards, and the con- 

 stant presence of a bright pale yellow or colourless bead in the outer end of their inner 

 segment, establish their identity and conic nature. As their stoutness decreases from 

 the periphery to the centre of the retina, each successive unit of surface in this direction 

 contains more cones ; and since each cone has its outer granule associated with it, the 

 numerical increase of the cones is attended with a corresponding increase in the number 

 of the outer granules. The inner ends of the cones pass through apertures in the mem- 

 brana limitans externa, which in its structure and relations resembles that of other 

 reptiles. 



2. The Outer Granule-Layer. — The so-called " granules" are large, round, and roundly- 

 oval nuclei, about -s^^' in diameter. In the periphery of the retina, where the cones 

 are stout, they lie in a single tier, one in the inner end of each cone, in or slightly 

 inside the plane of the membrana limitans externa ; while towards the fovea, where the 

 slenderness of the cones does not allow them to enclose their outer granule, the granules 

 form a separate layer, and each is connected with its cone by an intermediate fibre, con- 

 tinuous with the inner end of the latter. These fibres, for which I would suggest the 

 term " primitive cone-fibres," are identical with those I have described as coming off 

 from the inner end of the rods and cones in other reptiles, but their course and arrange- 

 ment in the chameleon are peculiar. 



3. Cone-fibre Plexus. — In the periphery of the retina, where the cones enclose their 

 outer granule, the primitive fibres, on leaving the inner end of the cones, curve verti- 

 cally inwards and combine in bundles, which run for a considerable distance hori- 

 zontally towards the periphery, and then again bend vertically inwards among the 

 inner granules. In the central region, where the cones are slender and numerous and 

 the outer granules lie in several tiers, the primitive cone-fibres first bend inwards from 

 the cones, next take up their outer granule, and then, at the inner surface of the outer 

 granule-layer, combine in bundles which unite in a plexiform manner to form a layer 

 that attains a remarkable thickness at the border of the fovea centralis. The inclina- 

 tion of the bundles, and their combination in the plexus, is such that the outer bundles 

 are gradually brought to the inner surface of the plexus. Here they are again resolved 

 into primitive fibres that curve inwards through the intergranule-layer to the inner 

 granules (fig. 45). H. Muller relates that in his hardened preparations he could not 

 determine whether the finer fibres were merely stuck together in the bundles of the 

 plexus, or whether divisions took place ; my belief is that they cohere very closely 

 in the bundles of the plexus, but that they never fuse or split. Lying between the 

 outer and the inner granules, this plexus was called by Muller the intergranule-layer. 



