10 bulletin: museum of comparative zoology. 



in depth and* bulk. To this stage are assigned all those fishes which, in 

 a strictly lateral view from either side, exhibit only one eye. The shorter, 

 proportionately deeper, larv;« metamorphose when they reach 8 or 9 mm. 

 in length. The degree of symmetry can better be seen in a front view 

 (Fig. 4) of a fish 4 mm. long, the only trace of asymmetry at this stage 

 being the slight elevation of the left nasal pit and the lack of absolute 

 bilateral symmetry in the shape of the mouth. The upper lip is slightly 

 drawn upward on the right side directly opposite the right nasal pit 

 {fv. olf.). 



Stage III. (Fig. 2) has been made to include those fishes in which the 

 eye of the blind side had so far migrated as to be visible when tlie fish 

 was viewed in profile from the ocular side. At this stage the eye lies in 

 the median plane in a depression immediately in front of the dorsal fin, 

 which has grown forward since the preceding stage. There is also a 

 noticeable change in the direction of the urostyle ^ (iir'stl.). 



In the last stage, IV., the eye has completed its migration, and, so far 

 as regards the distortion of the head, the fish is essentially in the adult 

 condition. Changes after this are merely accentuations of what is 

 found here. Figure 6 shows the dorsal tin {pin. d.) at this stage 

 extending as far forward as the middle of the eye. On the body are 

 to be seen the beginnings of the pigment areas which later color the 

 right side of the fish. 



The sinistral fish, Bothus, is at first symmetrically pigmented. The 

 lower side does not become colorless until the disappearance of the first 

 color pattern and the establishment of the much lighter adolescent 

 color, which comes after the turning. P. amcricanus, on the contrary, 

 is essentially non-pigmented until it is ready to become a bottom feeder. 



The front view of P. araericanus at this stage (Fig. 5) — the com- 

 pletely turned fish — is most instructive in bringing out the want of 

 symmetry. The left eye has moved through an arc of about 115 

 degrees, as may be seen by comparing this view Avith that of Stage II. 

 (Fig. 4). The left nostril has moved dextrad and dorsad, as if in the 

 passage of the eye it, too, had become involved. The angle of the 

 mouth on the right side bends sharply ventrad ; and the upper lip 

 of the right side is apparently drawn dorsad toward the right nasal 

 pit. From this point the mouth opening has the form of a long slit 

 which extends to the left and ventrad in a nearly straight line. 



In Paralichthys oblongus and in Bothus the mouth remains nearly 

 horizontal and symmetrical. 



1 For the development of the caudal fin of the flounder, see Agassi/, ('78). 



