WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. Ly 
the space through which later the eye must pass. Figure A is from 
a photograph of the model of the front part of the cartilaginous cra- 
nium of a 3.5 mm. fish, viewed obliquely from the front, the right side, 
and above. The line of vision makes an angle of about 30 degrees with 
the horizontal plane. Meckel’s cartilage no longer forms a simple bow 
lying in the horizontal plane. The anterior end is curved slightly ven- 
trad, and the bar of either side in passing backwards bends sharply 
ventrad to join, nearly at right angles, a series of cartilaginous masses 
(Fig. A hy-md.) representing the future quadrate, articular, symplectic, 
and hyomandibular bones. In cross section these cartilaginous masses 
have, in general, the form of an elongated oval, the axis of which in- 
clines dorsad and mesiad ; the ventral margin is slightly thicker than 
the upper. The space occupied by each separate cartilage in this series 
is not indicated in tl models, though in the sections the boundaries 
can be determined by the presence of the connective-tissue sheaths which 
limit the cartilages. 
The pterygo-palatine bars (pt-pal.) extend ventrad and caudad from 
each side-of the ethmoid to the quadrate region (compare also Fig. 10). 
At this stage the fish has a very small gape. The hyoid and gill-arch 
cartilages are present in their general shape, occupying most of the space 
between the right and left hyomandibular-quadrate masses, and ending 
in front just beneath the body of the ethmoid in the basi-hyal (da-hy). 
From the ethmoid mass arise also the supraorbital bars. These, in 
the salmon, extend backward from the ethmoid, curving upward and 
outward above the eyes, to the heavy cartilaginous mass of the otic cap- 
sules. In the flatfish of this stage, as shown in the reconstruction, 
there is but one complete supraorbital bar (the right), the left being 
represented by two remnants, an anterior and a posterior ; the anterior 
(trb. sword. s. a.) is @ process extending backward from the dorsal left- 
hand corner of the ethmoid; the posterior (trd. su’orb. s. p.) extends 
forward from the left otic capsule. It is through the space between 
these two projections that the left eye migrates. While, as yet, there 
is no external sign of an asymmetrical position of the eyes, internally 
preparations for such a condition are clearly established, for the middle 
portion of the left supraorbital bar has disappeared. 
I have sectioned only a few individuals of P. americanus in which the 
left supraorbital bar is still continuous, and even in them at the region 
corresponding to a transverse plane passing through the middle of the 
two eyes the bar is so reduced in thickness as to show in cross section 
only one or two cartilage cells. 
VOL. XL.— No. 1 2 
