WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONEGTES. 37 



braiii and eyes farther advanced, and Figure 1 1 (from an adult) shows 

 it completed. 



In the essentially adult condition of Stage IV., as shown in a front view 

 of the modelled brain and optic nerves (Figure F), the left eye has passed 

 so far to the right side that, taking into consideration the high degree of 

 mobility of the eye its field of vision almost coincides with that of 

 the right eye. The optic nerves curve still more in their passage 

 from chiasma to eye, and the distance is proportionately greater. 

 The right cerebral lobe (c6. dx.) is seen in the figure between the eyes, 

 and the left cerebral lobe {cb. s.) is seen on the right, behind the left 

 eye, and below the tectum. The left olfactory lobe is covered by the 

 left eye, but the right olfactory lobe — modelled as a continuation for- 

 ward of tlie right cerebral lobe — is seen between the two eyes. The left 

 optic lobe {tct. opt. s.) in both these instances (Figures £ and F) extends 

 farther anteriorly than the right. This is seen in the dorsal view of 

 the brain (Fig. 8). This figure also shows why in making cross-sec- 

 tions the left lobe of the cerebrum is cut before its olfactory lobe in 

 case one begins at the anterior end. 



The optic nerve — round in cross-section in the larvse — becomes 

 thrown into folds in the adult (Plate 5, Fig. 24). This condition is also 

 figured by Studnicka ('97) for one of the Pleuronectidse. The cross-sec- 

 tion may show as many as six or seven folds closely pressed together. 

 Small neuroglia nuclei are scattered throughout the length of the nerve. 



3. The Chiasma and Tracts with related Ganglia. 



The optic crossing is complete as in all teleosts. There is no inter- 

 lacing of fibres, as can be seen in Figure 19 (Plate 4), which is from a 

 fish in Stage IV. This is an approximately transverse section, which, 

 however, cut the left side of the fish sdmewhat farther caudad than it 

 did the right side. The plane of the section also inclines a little back- 

 ward and upward, so that it coincides with the plane of the anterior part 

 of the left optic tract, which slants in Figure 19 backward and upward 

 on its way to the tectum. The right tract is cut crosswise, nearly at 

 right angles to its course. (This is by mistake lettered ?«. opt. s. in 

 Figure 19. Of course, as it is posterior to the chiasma, it should have 

 been labeled trt. opt. dx. For the second section anterior to this the 

 label n. opt. s. would be correct.) The median, dorsal portion of the 

 tract (trt. opt. d.) passes upward through the nidulus corticalis (to be 

 described later) on its way to the median portion of the tectum. The 



