230 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



ventral as well as those with that nerve dorsal, and that, when 

 metamorphosis sets in, only those whose migrating eyes had dorsal nerves 

 survived. Unfortunately there is no evidence in favor of this view and 

 much against it. Williams, whose paper (:02) I iiave already quoted, 

 informs me that in the two species of Pleuronectidae studied by him all 

 the symmetrical young had the same type of optic nerve crossing that 

 the metamorphosed individuals had. I have myself determined the 

 positions of the nerves in the chiasmata of ten newly hatched but un- 

 metamorphosed Pseudopleuronectes americanus, and in all, the left 

 nerve was dorsal, as was characteristic of the adult. I therefore believe 

 that the young Pleuronectidae are hatched with tlie type of optic nerve 

 crossing characteristic of the adult, and that this may be looked upon 

 as an adaptation preparatory to the migration of the eye. 



Writers in the past, and even recent writers, such as Cunningham 

 ('90, p. 51) ; and Williams ('02, p. 1), often refer to the newly hatched 

 Pleuronectidae as "perfectly symmetrical" and with "eyes and all 

 other parts of the head ... as symmetrical as in any other fish." But 

 the way in which the optic nerves cross sets this question in a somewhat 

 different Hght. The soles, so far as their optic chiasmata are concerned, 

 doubtless are hatched in a condition like ordinary fishes, but those 

 Pleuronectidae that turn in one direction only come from the egg witli 

 a monomorphic type of nerve crossing that conforms in a mechanically 

 advantageous way to the ultimate direction of their turning. It is doubt- 

 ful whether the term symmetrical should be applied to the conditions of 

 the optic chiasmata of ordinary teleosts, but if it is so applied, the young 

 Pleuronectidae are not in that sense symmetrical, for of the two kinds of 

 chiasmata found in each species of ordinary teleosts only one occurs in 

 each species of Pleuronectidae, and this condition is established some 

 time before hatching. 



It might be inferred from what has gone before that the factors that 

 determine which eye in the Pleuronectidae will migrate are to be sought 

 for, not, as is usually done, in the environment when the young fisli 

 undergoes its metamorphosis, but in the egg at the time when the optic 

 chiasma is established, or even earlier. ~ But this assumption would 

 imply that the manner of the crossing of the optic nerves and the mi- 

 gration of the eye are mutually dependent phenomena. That they are 

 not invariably so can be shown by the following observations. 



A few species of Pleuronectidae are represented by both sinistral and 

 dextral individuals. Thus Pleuronectes platessa, a dextral species, may, 

 according to Duucker ('96, p. 83) be occasionally represented by a 



