WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 17 



the space through -which hater 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 the models, though in the sections the boundaries 

 can be determined by the presence of the connective-tissue sheaths which 

 limit the cartilages. 



The pteryffo-pcdatine bars (p(-pal.) extend ventrad and caudad from 

 each side of the ethmoid to the quadrate region (compare also F'ig. 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 (ba-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 riglit), the left being 

 represented by two remnants, an anterior and a posterior ; the anterior 

 (trb. siCorb. s. a.) is a process extending backward from the dorsal left- 

 hand corner of the ethmoid ; the posterior {trb. suorb. 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. 12 * 



