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 have 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 the 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 light. 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 with 
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 fish 
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 Duncker (96, p. 83) be occasionally represented by a 
