WILLIAMS: MIGRATION OF EYE IX PSEUDOPLEURONECTES. 33 



Later the ect-ethmoid of the " blind " side is formed as an ectostosis 

 around the cartilage of that wing of the ethmoid and sends back a 

 process along the line which the supraorbital cartilage had occupied. 

 This meets and fuses with a forward process of the frontal of that side, 

 thus forming the "Brlicke," which becomes in the adult fish the most 

 voluminous bony support of the nasal region. 



The supraorbital of the other side keeps its connection with the ear- 

 capsule much longer. Since the non-migrating eye moves downward to 

 only a slight degree, the sspraorbital has small space for movement to 

 evade the pressure of the tissues in front of the migrating eye. So we 

 find, in the latest stages in which this supraorbital appears at all, that 

 the structures of the median plane have been crowded over upon the 

 supraorbital and that this now appears as the cartilage " hook " (ham. 

 eth., Fig. JD), which extends backward between the eyes and is at this 

 time the chief tissue separating them. 



In Bothus each frontal bone, when formed, sends forward a slender 

 process between the eyes, but in P. americanus the process arises from 

 the frontal of the ocular (right) side only. 



V. The Optic Portion of the Central Nervous System. 



1. General Condition in the Adult. 



If the brain of the cod be taken for comparison, the axis of the cerebro- 

 spinal part of the nervous system of P. americanus shows bendings that 

 seem not to exist in the cod. There is in the spinal cord a bend which 

 is convex upward (dorsad) and is apparently induced by the size of the 

 digestive organs. In front of this, in the region of the medulla, occurs 

 a bend which is convex ventrad (Plate 1, Fig. 6). Finally there is 

 also a decided bend whicli is convex towards the eyeless side (Plate 2, 

 Fig. 11). The muscles of the eyeless side being less developed, that side 

 is more nearly flat than the oculai- side, which is convex. 



Figure 8 (Plate 2) is a dorsal view of the brain of a fish (P. ameri- 

 canus) three inches long. The curves mentioned are not yet empha- 

 sized. An evident sign of asymmetry is seen in the inequality in the 

 size of the olfactory lobes, that of the right side being much the larger. 

 This lobe may, in the adult, have six times the volume of that of the 

 left side (compare Fig. 11). The relative sizes of the lobes of the cere- 

 brum is different in different individuals. In the specimens shown in 

 Figures 8 and 9 (Plate 2) and in Figure F (p. 36) the left lobe is the 

 larger ; but in a number of adult fishes the right lobe was the larger. 



VOL. XL. NO. 1 3 



