WILLIAMS: MIGKATION OF EYE IN PSEUDOPLEUKONECTES. 49 



a fish just hatched "anticipating the twisting and subsequent unequal 

 development exhibited by the head of Pleuronectids." Those larvae 

 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 distancb 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. 



1 4. The optic nerve of the migrating eye is 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 geniculnte body. 

 No other bundles of fibres leave the tract before it reaches the tectum. 



17. The ganglia habenulae are symmetrical, at least in the larva 

 before metamorphosis. 



VOL. XL. — NO. 1 4 



