WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 49 
a fish just hatched ‘anticipating the twisting and subsequent unequal 
development exhibited by the head of Pleuronectids.” Those larve 
which remain pelagic until better able to compete at the sea bottom 
become the adults which fix the time of metamorphosis on their progeny. 
VII. Summary. 
1. The young of Limanda ferruginea are (probably) in the larval stage 
at the same time as those of Pseudopleuronectes americanus. 
2. The recently hatched fish, both P. americanus and Bothus, are 
symmetrical, except for the relative positions of the two optic nerves, 
3. The first observed occurrence in preparation for metamorphosis in 
P. americanus is the rapid resorption of the part of the supraorbital 
cartilage bar which lies in the path of the eye. This is probably due to 
pressure from the migrating eye. 
4. Correlated with this is an increase in the distance between the eyes 
and the brain, caused by the growth of the facial cartilages. 
5. The migrating eye moves through an arc of about 120 degrees. 
6. The greater part of this rotation (three-fourths of it in P. ameri- 
canus) is a rapid process, taking not more than three days. 
7. The anterior ethmoidal region is not so strongly influenced by this 
twisting as the ocular region. 
8. The location of the olfactory nerves shows that the morphological 
mid-line follows the inter-orbital septum. 
9. The cartilage mass lying in the front part of the orbit of the adult 
eye is a separate anterior structure in the larva. 
10. With unimportant differences, the process of metamorphosis in 
the sinistral fish is parallel to that in the dextral fish. 
11. The original location of the eye is indicated in the adult by the 
direction first taken, as they leave the brain, by those cranial nerves 
having to do with the transposed eye. 
12. The only well-marked asymmetry in the adult brain is due to 
the much larger size of the olfactory nerve and lobe of the ocular side. 
13. There is a perfect chiasma. 
14. The optic nerve of the migrating eye is s always anterior to that of 
the other eye. 
15. The optic tract is divided into dorsal and ventral portions. 
16. There are fibres from the tract which enter the geniculate body. 
No other bundles of fibres leave the tract before it reaches the tectum. 
17. The ganglia habenule are symmetrical, at least in the larva 
before metamorphosis. 
VOL. XL. —No. 1 4 
