WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONECTES. 25 



There is also indicated at this stage a beginning of the forward rotation 

 of the dorsal margin of the ect-ethnioid cartilages about a transverse axis 

 passing through them. The end of the bristle (Fig. G) over the trabec- 

 ule cranii is, therefore, not greatly posterior to the outer end, which is 

 seen against the left pterygo-palatine as a background. The final result 

 of this rotation of the ect-ethmoids about the axis connecting them is to 

 make the axes of both foramina transverse instead of longitudinal. Con- 

 sequently in an oblique view from the right side, as in Figure D, one is 

 looking at the olfactory foramina from that face of the ect-etiimoids 

 which at an earlier stage (Figs. A, B) was directed posteriad. Instead, 

 therefore, of seeing the ends of the olfactory nerves wliich are distal to 

 the foramina, as would be the case if the cartilages were viewed from 

 the same direction at an earlier stage (Figs. A, B, and C) one would 

 now see the'iv proximal ends. 



A twisting of the ethmoids (in a clockwise direction when viewed 

 from behind) about the antero-posterior axis of the fish, greater than is 

 indicated in Figure C, results in the further elevation of the ect-ethmoid, 

 olfactory foramen, and pterygo-palatine of the left side, while the supra- 

 orbital, the ect-ethmoid, the olfactory foramen and the pterygo-palatine 

 of the right side are correspondingly depressed. 



e. Stage IV. 



The oldest facial region modelled (Fig. D) — that of a small fish 

 (Plate 1, Figs. 5, G) having the eyes in the adult position — represents 

 my Stage IV. 



The eyes are located one on each side of the flat hook-like plate of 

 cartilage (Fig. D, ham. eth.) which, with the previously mentioned 

 median arch {arc. eth. m.), runs back along the morphologically median 

 plane (the plane between the eyes). Tlie interorbital septum of con- 

 nective tissue is continuous with these^two cartilaginous processes, filling 

 the space between them and extending thence backward. That this 

 occupies the morphologically median plane, is proven by the position of 

 the olfactory nerves, which lie one on each side of this septum. Ante- 

 riorly the left nerve passes through the opening (for. olf. s.) seen in the 

 left (now upper) wing of the ethmoid and ends in the nasal capsule, 

 which lies immediately in front of it. The right nerve comes from be- 

 low the hook-shaped cartilage and passes through a foramen (for. 

 olf. dx.) in the anterior part of the ethmoid to the right nasal capsule, 

 which is located somewhat in front of the ethmoid and near the anterior 

 end of the right pterygo-palatine. 



