14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



dermal bones. The previously upper side of the eye now lies on the in- 

 terorbital sej^tum, therefore most ventral; whereas the previously lower 

 side of the eye is now near the dorsal fin, therefore highest. The eye 

 has thus rotated 180 degrees. The side of the migrating eye that is 

 turned toward the blind side of the head is now closed in by the forma- 

 tion of new dermal'bones. The socket is completely open in the region 

 of the optic nerve. By the migration of the eye, the anterior oblique 

 eye nmscles, which arise from the hinder border of the ethmoid, are laid 

 bare ; a thin covering of dermal bone grows over these also. The wing 

 of the ethmoid on the eyeless side, is fused to a part homologous with 

 the supraorbital cartilages ; these grow upward and inward, the latter 

 helps in forming the anterior wall of the new orbit. 



PfefFer says that, though the ossification is a continuous process, one 

 may distinguish, if he will, three stages in the development of the paro- 

 stotic cranial bones of fishes, characterized by — 



(1) The first delicate osseous investment of the cartilage ; 



(2) The dermal ossification which establishes approximately the per- 

 manent forms of the bone ; 



(3) The ridges, crests, wings, and the like, — entirely superficial addi- 

 tions, — which are probably always connected with muscular action. 



In the flounder the rotation begins while the frontal region of the 

 young fish is in the first of these stages. Soon the frontal (cartilaginous) 

 is in quite another place, under quite another region of the skin. When 

 it has changed its position, there is dermal bone produced over it in its 

 new position ; but there is not the least reason why the skin under which 

 it would normally have lain should suddenly lose the power of producing 

 bone, — and in fact it does not, for it produces the bridge. The bony 

 bridge, then, is the parostotic ossification of a precise region of the cutis, 

 and if the cranium had remained symmetrical, it would have fused to 

 the frontal ; but inasmuch as there is a displacement of the region of the 

 (cartilaginous) skull, this dermal ossification has become attached to 

 those bones which took a position directly beneath this bone-producing 

 region of the cutis after the displacement of the (cartilaginous) skull. 



Pfeffer's final paper, so far as I know, has not yet appeared ; but in a 

 short note ('94) the author states again that the interorbital septum 

 twists on its long axis, and adds: (1) that the migrating eye, when it 

 reaches the mid-line, loses the thin patch of skin which has separated the 

 cornea from the outer world, and (2) that the dorsal fin, the muscles 

 and the bones develop along the physiological axis of the body, the con- 

 tiniuition of the sftinal column. 



