WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONECTES. 21 



median region of the ethmoid, thus bridging over the left ethmoid notch 

 and leaving between the mes-ethmoid and the region of the anterior end 

 of the right supraorbital cartilage an orifice {for. olf. s.), which corre- 

 sponds to the notch on the right. In other specimens I find that both 

 wings of the ethmoid have sent out processes to fuse with the mes- 

 ethmoid, thus converting both notches into foramina for the passage of 

 the olfactory nerves to their capsules on the front of the ethmoid. 



In this model a bent wire is inserted into the mes-ethraoid in the median 

 plane to aid in locating the position of that plane, — the plane in which 

 the future interorbital septum is to develop. There is as yet no trace 

 of this septum in the specimen modelled ; but Figure 18 (Plate 4) shows 

 a cross section of the head of a fish (P. americanus) of this stage, which 

 does indicate the position of the future interorbital septum. The fine 

 vertical lines outside the figure represent the projection of the sagittal 

 plane of the fish. A small bar of cartilage (arc. eth. m.) is seen in cross 

 section above the mes-ethmoid. Traced anteriorly a few sections, this 

 fuses with the ethmoid. Traced posteriorly it soon unites with the 

 thin fused trabecular cranii not far from where they pass over into the 

 ethmoid. It is, then, a slanting bar, or arch, from near the anterior 

 end of the trabeculcC cranii to the posterior face of the ethmoid. In an-, 

 other specimen (Figure C", p. 24) this arch has become larger and ap- 

 pears as the forwai'd prolongation of the trabeculise (trb.). In the space 

 beneath this arch lie the oblique eye muscles, two of which (the right 

 and left inferior oblique) appear in Figure 18. The same figure shows 

 that the migrating eye may exert pressure directly on the cartilage, for 

 the left eye-ball is indented by the left wing of the ethmoid. 



In another specimen of this stage, which had lost the migrating eye 

 in the process of turning, there were certain peculiarities worthy of con- 

 sideration. This fish, too, had a well-developed median arched cartilage 

 on the posterior f;%ce of the ethmoid. "The right superior oblique muscle 

 had its origin at the angle produced by the junction of the arch and the 

 body of the ethmoid. The inferior oblique was attached lower, at the 

 angle made by the union of the ethmoid and the trabeculse. The pos- 

 terior face of the ethmoid is the usual place of attachment for these 

 muscles, though a specimen of B. maculatus had both the inferior and 

 superior oblique muscles attached on the median arched bar. The most 

 noticeable peculiarity of this specimen was shown in the origin of the 

 supraorbitals. As I have said, there was no eye present on the left 

 side. The anterior end of the left supraorbital bar still persisted in this 

 specimen in the form of a stub projecting backward and slightly upward 



