WILLIAMS: MIGRATION OF EYE IN 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 ‘Briicke,” 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 supraorbital 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. D), 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 which 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 ocular 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 
