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 oneeye. The shorter, 
proportionately deeper, larvae 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 the 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’ (wr’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 fin (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. americanus, on the contrary, 
is essentially non-pigmented until it isready to become a bottom feeder. 
The front view of P. americanus 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 with 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 Agassiz (’78). 
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