14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
dermal bones. The previously upper side of the eye now lies on the in- 
terorbital septum, 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 muscles, 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 ina 
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- 
tinuation of the spinal column. 
a 
