39 
2. The retina. 
(a) There is more variety in the degree of development of the pig- 
ment epithelium than in any other structure of the eye. Ritter has found 
that in Typhlogobius this “layer has actually increased in thickness con- 
comitantly with the retardation in the development of the eye, or it is 
quite possible with the degeneration of this particular part of it. * * * 
An increase of pigment is an incident to the gradual diminution in fune- 
tional importance and structural completeness.”’ There is so much varia- 
tion in the thickness of this layer in various fishes that not much stress 
can be laid on the absolute or relative thickness of the pigment in any one 
species as an index of degeneration. While the pigment layer is, relative 
to the rest of the retina, very thick in the species of Chologaster, it is 
found that the pigment layer of Chologaster is not much, if any, thicker 
than that of Zygonectes. Exception must be made for specimens of the 
extreme size in papilliferus and agassizii. In other words, primarily the 
pigment layer has retained its normal condition while the rest of the 
retina has been simplified, and there may even be an increase in the 
thickness of the layer as one of its ontogenic modifications. Whether the 
greater thickness of the pigment in the old Chologaster is due to degenera- 
tion or the greater length of the cones in a twil‘ght species, I am unable 
to say. In Typhlichthys, which is undoubtedly derived from a Cholo- 
gaster-like ancestor, no pigment is developed. The layer retains its epi- 
thelial nature and remains apparently in its embryonic condition. It may 
be well to call attention here to the fact that the cones are very sparingly 
developed, if at all, in this species. In Amblyopsis, in which the degenera- 
tion of the retina has gone further, but in which the cones are still well 
developed, the pigment layer is very highly developed, but not by any 
means uniformly so in different individuals. The pigment layer reaches 
its greatest point of reduction in rosae where pigment is still developed, 
but the layer is fragmentary, except over the distal part of the eye. We 
thus find a development of pigment with an imperfect layer in one case 
and a full developed layer without pigment in another, Typhlichthys. In 
the Chologasters the pigment is in part prismatic; in the other species, 
granular. The rods disappear before the cones. 
(b) In the outer nuclear layer a complete series of steps is observa- 
ble from the two-layered condition in papilliferus to the one-layered in 
cornutus to the undefined layer in Typhlichthys and the merging of the 
