40 
nuclear layer in Amblyopsis, and their occasional total absence in rosae. 
The rods disappear first, the cones long before their nuclei. 
(c) The outer reticular layer naturally meets with the same fate as 
the outer nuclear layer. It is well developed in papilliferus, evident in 
cornutus, developed in spots in Typhlichthys, and no longer distinguish- 
able in the other species. 
(d) The layers of horizontal cells are represented in papilliferus by 
occasional cells; they are rarer in cornutus and beyond these have not 
been detected. 
(e) The inner nuclear layer of bipolar and spongioblastic cells is well 
developed in papilliferus. In cornutus it is better developed in the young 
than in the older stages where it forms but a single layer of cells. There 
is evidently in this species an ontogenic simplification. In the remaining: 
species it is, as mentioned above, merged with the outer nuclear layer into- 
one layer which is occasionally absent in Troglichthys. 
(f) The inner reticular layer is relatively better developed than any 
of the other layers, and the conclusion naturally forces itself upon one 
that it must contain other elements besides fibres of the bipolar and gan- 
glionic cells, for, in Amblyopsis and Troglichthys, where the latter are very 
limited or absent, this layer is still well developed. Horizontal cells have 
only been found in the species of Chologaster. 
(g) In the ganglionic layer we find again a complete series of steps 
from the most perfect eye to the condition found in Troglichthys. In 
papilliferus the cells form a complete layer one cell deep, except where 
the cells have given way to the optic fibre tracts which pass in among the 
cells instead of over them. In cornutus the cells have been so reduced in 
number that they are widely separated from each other. With the loss 
of the vitreous cavity the cells have been brought together again into a 
continuous layer in Typhlichthys, although there are much fewer cells. 
than in cornutus even. The next step is the formation of a solid core of 
ganglionic cells, and the final step the elimination of this central core in 
Troglichthys, leaving but a few cells over the anterior face of the retina. 
(h) Miillerian nuclei are found in all but Amblyopsis and Troglichthys. 
In C. cornutus they lie in part in the inner reticular and the ganglionic 
layer. Cells of this sort are probably also found among the ganglionic 
cells of Typhlichthys. 
3. The optic nerve shows a clear gradation from one end of the series 
of fishes to the other. In Chologaster papilliferus it reaches its maximum 
