WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 31 
in Bothus the dermal frontal is not yet present in the region through 
which the eye passes, and therefore cannot be resorbed. At Stage IV., 
2. e., after the migration is practically completed, there is to be found in 
P. americanus under the surface of the skin behind the eye region a thin 
plate of bone, which I take to represent the left frontal. The supra- 
orbital cartilage of the side from which the migrating eye comes lies in 
the region to which Pfeffer assigns the degenerating frontal in his 
species, and we have seen that this bar is resorbed. Perhaps in his 
species the dermal bone (frontal) is formed relatively earlier than in 
P. americanus. 
Pfeffer’s statement that the transposition of the eye is accompanied 
by a rotation on its own axis through an are of 180 degrees is not quite 
correct for our species. The arc in P. americanus varies slightly in dif- 
ferent individuals, but is approximately 120 degrees. 
Neither will his theory of the formation of the ‘‘ Knochenbritcke”’ fit 
the facts in Pseudopleuronectes. His argument (p. 8) is that when the 
frontal bone of the blind side changes its position, dermal bone is pro- 
duced, not only over it in its new position, but also in the region of the 
integument beneath which the frontal was originally located, the latter 
dermostosis being known as the “ Brticke.” In our species at least, the 
frontal, when once formed, does not change its position. So its onto- 
genetic location does not explain the formation of the “ Briicke.” 
In Pfeffer’s more recent paper (’94) he states, as before, that very 
young symmetrical Pleuronectide have cartilaginous crania. The “ In- 
terorbitalbalken ” [ Interorbital-Decke ?] twists on its long axis, its dorsal 
edge toward the future ocular side. One eye moves downward while the 
other comes to lie upon the “ Interorbitalbalken.” If any sheathing 
bone is already formed on the “ Interorbitalbalken,” the elevated eye 
resorbs the part of the bone which is in its way. Then, on the side of 
the upper eye corresponding to the blind side of the adult fish there 
is formed a bony orbit, which fuses with the gradually developing dermal 
bones, so that the skull of such an individual leaves the false impression 
that the eye has traversed some of the bones of the skull. 
The upper eye does not, according to Pfeffer, travel around to the 
other side of the skull, but ascends only a little, until on a level with the 
part of the skull between the eyes ; however, from this time forward it 
looks in the direction of the ocular side. At the same time the thin 
piece of skin (“ Kérperhaut”) now separating the cornea from the outer 
world, disappears. 
In regard to the last point, I may say that in both species I find a 
