132 American Fisheries Society 



have an average length of 6.5 mm. and an average depth 

 of 2.75 mm. Most of the characteristics of the adult are 

 present. The eyes, as shown in the figure, have taken 

 the adult position; both are now functional on the right 

 side. The caudal fin is practically separated from the 

 dorsal and ventral fins but in most cases a slight remnant 

 of the old finfold can still be observed. The fin rays of 

 the dorsal and ventral fins are sufficiently developed to 

 give a broken margin to the fins. The pigment is much 

 more diffuse than in the earlier stages. The brain and 

 spinal cord are now distinctly outlined superficially by the 

 pigment spots over them. The fish is at this time very simi- 

 lar to the adult except for the asymmetry of the mouth. 

 The mouth at this stage is symmetrical and in fact re- 

 mains so for a considerable period. Only a slight degree 

 of asymmetry can be seen in a fish of several centimeters. 



In the later stages there is a gradual loss of pigment 

 on the left side and a gradual increase of pigment on the 

 right side. In a fish 8 mm. long the upper or dextral 

 side is completely pigmented while the left side has lost 

 its pigment with the exception of a few scattered spots, 

 about twenty in number, in the region of the snout. At 

 what time these spots are lost I do not know. They are 

 entirely absent in a fish of 20 mm. and are probably lost 

 considerably earlier. 



Behavior of the young fish. There has been much 

 speculation as to the cause of the turning of the flat- 

 fishes and the relation of the migration of the eye to the 

 question of turning. In regard to this I can only say that 

 I learned nothing in my study of the fishes through the 

 period of metamorphosis that threw any light on the 

 question. The study of the segmentation of the egg in 

 other genera has added nothing. I think it would be in- 

 teresting and possibly enlightening if the chemicals that 

 produce the Cyclopean eye in the "normal-eyed" fishes 

 were applied to the flat-fishes. I am convinced that the 

 migration of the eye is but an external manifestation of 

 the turning and is in no way the cause of the turning. 



