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 R. 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 thickenning 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., 
z., 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 
