WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEUEONECTES. 35 



by a few twigs only (V. opt. j^fiid., Fig. 11). The left superior ophthal- 

 mic of the flatfish {V. opt. su.), after emerging from the skull with the 

 rest of the fifth nerve, as in the cod, runs from left to right (Fig. 11) 

 through the passage formed by the "Brucke," which results from the 

 fusion of the posterior angle of the pre-frontal and the corresponding 

 anterior angle of the left frontal. It then takes the regular median path 

 between the eyes to its distribution on the snout. The bone is formed 

 around the nerve in its new position after the migration of the eye. 



The seventh nerve in both the cod and the flounder emerges from the 

 skull with the fifth. The ninth in the cod lies between the two chief 

 roots of the tenth, with which it passes out. In the flounder the ninth 

 nerve lies in front of the tenth and passes through the ear capsule to its 

 distribution on the hyoid and first gill arch. 



2. The Optic Nerves. 



In the cross-section of a fish in Stage I. (Plate 3, Fig. 1 7), one section, 

 lOyu, thick, contained the whole length of both optic nerves from the 

 blind spot to the chiasma. The blind spot is very near the outer ventral 



ch. dx. 



id. opt. 



_ oc. mig. 



A precisely front view of tlie fore part of the brain, the optic nerves and 

 a portion of each of the optic cups, modelled in wax (Born's method) 

 from a specimen in Stage III. X 50. 



For explanation of lettering, see Abbreviations under Explanation of 

 Plates. 



edge of the retina and in about the middle of the eye antero-posteriorly. 

 Therefore the chiasma is in the transverse plane which passes through 

 the middle of the eyes. There is, as yet, scarcely any want of symmetry, 

 the left eye being only slightly higher than the right. 



