164 PROCEEDINGS OF THE ACADEMY OF [1894. 



to suggest the complete homology of a group consisting of the five 

 central rod- cells and their four surrounding double cones in the 

 retina of the salmon with a single ommatidium of the Arthropod eye. 



To what extent a similar condition holds in the arrangement of the 

 bacillary retinal elements in other groups of vertebrates I am not 

 prepared to say. But this remarkable regularity and repetition of 

 arrangement of cellular elements in the retina of a fish is at least 

 very suggestive. Blaue has shown that the olfactory organ of fishes 

 is made up in some cases of very small discrete sense-organs simula- 

 ting the structure of taste-buds and the small circular sensory buds 

 found in the lateral line organs of fishes. Ayers has traced the 

 probable origin of the complex sensory structures found in the cochlea 

 of vertebrates to a multiplication and subsequent coalescence of 

 originally discrete sense- buds or end-organs, that arose in a way 

 similar to the discrete end-organs of the lateral line. The detection 

 of regularly repeated groups of sense-cells in the retina of one of the 

 lower vertebrates thus becomes highly significant. Is it possible, 

 after all, that the retina of vertebrates has been evolved by the 

 coalescence of very small groups of sense-cells that were at one time 

 separate and simple sense-organs, just as the complex olfactory and 

 auditory sensory epithelia of vertebrates appear to have so arisen ? 

 The arrangement that has been described appears to extend through- 

 out the whole of the functional part of the retina of the larval 

 salmon, so that the grouping of the cells is at least uniform in this 

 type during the early stage of its existence. This is a feature that 

 is in strong contrast with that met with in the retinre of mammals, 

 in which the distribution of rods and cones is not uniform over the 

 whole functional surface of the organ. If, however, there is any 

 truth in the suggestion that the retina of the higher vertebrates has 

 arisen in the same way as the sensory epithelia of the olfactory and 

 auditory organs, namely by the coalescence of originally distinct 

 but repeated, similar sense-organs, then the arrangement that we 

 actually find in the retina of the salmon approximates such a primi- 

 tive condition of structure, though by no means as perfectly as is 

 seen in the compound eyes of Arthropods. 



It may be added that the reason that I assume such a regularly 

 repeated structure recurs throughout the whole of the retina of the 

 young salmon, is because it is found that whenever the planes of 

 meridional sections coincide with rows of double cones, the latter 



