WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONECTES. 9 



peculiar fish described by Schiodte was an example of an intermediate 

 method . 



Only two other descriptions of intermediate methods of eye-transition 

 need be noticed. Ehreubaum ('96) has discussed, among other points, 

 metamorphosis in the flatfishes of the German Ocean. Stages of the 

 larvae of the commoner species in which the eye passes around the head 

 are given. In the larva of Arnoglossus laterna, which strongly resembles 

 the so-called Plasrusise, the dorsal fin extends to the nostril while the 

 fish is yet symmetrical, so that the eye must pass under the dorsal fin as 

 in Plagusia. Tlie prolongation of the dorsal fin to the nasal pit and the 

 position of the right eye close to the lower margin of the fin (after 

 migration) prove, in Ehrenbaum's opinion, that the right eye is 

 shoved through imder the dorsal fin from the right to the left side. 



Recently a Japanese zoologist, T. Nishikawa ('9'), found a case 

 where the dorsal fin extended along the head as far as the end of the 

 snout in close contact with, but not fused to, the skin. There were no 

 fin rays located in the eye region. The right eye passed through a slit 

 between the fin and the head in one day, passing thus from one side 

 completely to the other. Unfortunately the fish died, so that it is not 

 known whether the fin would have fused later to the dorsal part of 

 the head or not. 



2. Description of Stages. 



For convenience of description four stages of development may be 

 recognized in Pseudopleuronectes americanus. 



Stage I., the recently hatched fish, is represented (Plato 1, Fig. 1) by 

 a specimen 3.5 mm. long and 12 days old. Owing to its wide dorsal 

 and ventral fins being so transparent as to be scarcely visible, the 

 living animal resembles, in its general appearance, a very minute 

 pin with an elongated head. It is" essentially symmetrical. I have 

 sectioned the eggs as well as the young fish and find a close resem- 

 blance to the figures given by Fullarton ('91) in his work on the develop- 

 ment of the plaice, Pleuronectes platessa, which is the nearest European 

 representative of our flatfish. His drawings, too, show the eyes to be 

 symmetrical in position. There are few pigment cells in the body of 

 an animal of this stage and they are arranged in much broken 

 longitudinal lines. 



The largest of the recently hatched fishes are nearly as long as the 

 smallest of the pelagic larvre (Stage II., Plate 1, Fig. 3), which were 

 taken the first of June ; but between the two there is a great diflTerence 



