WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 13 



ocular portion of the frontal of the eyeless side, the corresponding part of the 

 other frontal forms almost the entire external boundary of the orbit. 



" (6) The olfactory foramen and the place of suspension of the anterior sub- 

 orbital bone are further forward on the ocular side. . . . The articulation of the 

 epitympanic bone to the cranium, in the halibut and plaice, likewise extends 

 further forward on the ocular side. 



"(7) The axis of the keel of the cranium . . . points . . . to the eyeless side." 



PfefFer in a preliminary paper ('86) without illustrations, has described 

 the larval stages of development in one of the Pleuronectidae. As he is 

 the only writer who speaks of the conditions iu the interior of the bead, 

 his conclusions are given in some detail. 



The young fish has an entirely cartilaginous cranium, in which the 

 eye sockets are separated below by the sphenoid, and above by the inter- 

 orbital roof (Zwischenaugen-Decke) ; but between these the sockets com- 

 municate freely with each other. The ethmoid, constituting the anterior 

 part of the cranium, develops a wing on each side, the place where the 

 wings join the body of the ethmoid being marked by the presence of the 

 nasal openings. In very young animals the bulbi olfactorii are embraced 

 by the ethmoidal roof; but later they are forced backward behind it. 



Over the interorbital and ethmoidal regions runs a ridge-like dermal 

 bone, which is triangular in cross section, and stands vertically ; it sup- 

 ports the dorsal fin, and is at first free from the cranium. It is the 

 " principal frontal " of authors. 



In the second stage examined by Pfeffer, the migratory eye has risen 

 so that half of it is above the level of the interorbital roof. The brain 

 capsule remains unchanged, except that it has received the bulbus olfac- 

 torius, which has been forced backward by the migration of the eye. 

 The interorbital roof is bent outward toward the eye side and soniewhat 

 twisted on its long axis. At the same time the frontal, now grown fast 

 to the interorbital, makes with it a gfeat bend. However, only a broad 

 band — its basal portion — remains, while the greater, vertical part of it 

 is for the most part resorbed by the migrating eye. There now remains 

 between the migrating eye and the eye side only the translucent, thin 

 outer skin which previously covered the dermal bone. The front part 

 of the ethmoidal region is symmetrical ; but the upper part of the wing 

 of the eye side has fused to the fron to-orbital and is now continuous 

 with the developing supraorbital cartilage [bone?], while the whole rim 

 of the wing of the blind side remains free. 



The transposed eye at a later stage occupies a pit which opens up- 

 ward and toward the eye side and is surrounded by a high rim of thin 



