72 BULLETIN OF THE 



variations of functionless organs. Having now before us the facts relating 

 to the structure of the eye, we may pass to some reflections on their 

 significance when considered from a comparative and a developmental 

 point of view. 



First of all, I will speak of the pigment layer of the retina. This has 

 the greater interest since, according to E. Wright (Wiedersheim, '86, 

 p. 427), in the retina of the "blind tish Chologaster papilliferus there is 

 no pigmented epithelium." 



It has already been shown that in Typhlogobius this layer is always 

 thicker, relatively, than in the normal fish eye, being thicker than the 

 entire remaining portion of the retina. I am in considerable doubt as to 

 how this thiokenning has taken place. The first explanation that sug- 

 gested itself to me was that the choroid had become wholly converted 

 into pigment and fused with the pigment lamella of the retina. How- 

 ever, the dense and uninterrupted character of the pigment of the layer, 

 and the evenness of its external surface, at once threw grave doubts in 

 the way of this explanation, and the more because of the rather meagre 

 development of the choroid in the normal eye of bony fishes. Then, as 

 the choroid was found on further study to be present outside of this 

 layer, the only remaining alternative was to suppose the latter to be 

 wholly derived from the proximal wall of the primitive optic vesicle ; 

 i, e. to represent the pigment lamella of the retina. We may possibly 

 suppose that the proximal wall of the primitive optic vesicle never be- 

 came thinned out as it does in normally developing eyes ; but the fact 

 that this process takes place very early — in bony fishes, at least, by the 

 time the differentiation of the retina has begun — is quite a serious ob- 

 jection to such a supposition. But even if this were the case, it is hardly 

 possible to believe that this layer was ever as thick as we find the pig- 

 ment layer in the adult fish to be. We seem forced to suppose that for 

 some reason the layer has actually increased in thickness concomitantly 

 with the retardation in the development of the eye, or, it is quite pos- 

 sible, with the degeneration of this particular part of it. 



I would call attention to the comparison of Typhlogobius with Cleve- 

 landia in this regard. From the figure of the retina of the latter, it will 

 be seen that the retinal pigment appears in two quite well marked 

 layers, an outer and an inner, the two being connected at short but 

 somewhat irregular intervals by crossbeams or processes (Fig. 20, ex., 

 t., and m.). From this it seems that the inner extremities of the pro- 

 cesses of the retinal pigment layer, which in normal eyes, and particu- 

 larly in many teleostean eyes, project far down among the rods and 



