WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONECTES. 31 



in Both us the dermal frontal is not yet present in the region through 

 which the eye passes, and therefore cannot be resorbed. At Stage IV., 

 t. 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. 



PfeflFer's statement that the transposition of the eye is accompanied 

 by a rotation on its own axis through an arc 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 " Ivnochenbrtlcke " 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 "Briicke." In our species at least, the 

 trontal, 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 Pleuronectidte have cartilaginous crania. The " In- 

 terorbitalbalken " [Interorbital-Decke 1] 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 (" Korperhaut ") 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 



