WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 19 
tion of cartilage in the tail of the tadpole. In that case, according to 
Looss’s interpretation, it was the chorda sheath which restricted the 
diffusion of some of the products of the degenerating cells. He, too, 
found that the intercellular substance was the first to disappear in 
resorption. 
Whether the cartilage nuclei, when set free by the disintegration of 
the intercellular substance, degenerate completely, or join the nuclei of 
the connective tissue, I cannot determine. There is much resemblance be- 
tween the compact nuclei of degenerating cells and those of the sheath. 
Since the bar disappears first in the middle region, there are, for a 
short time, two degenerating regions, one which will end at the ethmoid 
and the other at the persistent stub in front of the ear capsule. The 
location of these will be evident by reference to Plate 2, Figure 10 (éréd. 
su’orb. s. a. and p.). 
When in P. americanus the frontal of the eyeless side is formed, its 
main body takes the position of this posterior stump of the left supra- 
orbital bar. It is significant that there isno more space provided by this 
degeneration than is barely necessary for the ready passage of the eye. 
The body of the ethmoid is very irregular in shape. Besides the two 
wings with which the supraorbitals are connected, there is a median 
elevation in the sagittal plane of the fish (ms’eth., Fig. A), and a forward 
knob-like projection (ert. orb. a.) in the same plane. The two olfactory 
pits lie just in front of the wings of the ethmoid, and the olfactory nerves 
pass to them through the two deep notches (v’cis. eth. dx. and s.) seen 
on the dorsal surface of the cartilage. The right nerve passes between 
the supraorbital bar of the right side and the median elevation ; the 
left nerve between the left supraorbital stub and the median elevation. 
In this left notch the superior oblique muscle of the left eye takes ¢ts 
origin, and in some cases the superior oblique muscle of the right eye 
has its origin also close to that of the left eye, therefore at the left of the 
sagittal plane. 
ce. Stage ITI a. 
Figure & is photographed from the model of the cartilages of a fish of 
Stage III. (Plate 1, Fig. 2), where the left eye could be barely seen pro- 
jecting over the top of the head as the fish lay on its left side. The left 
wing of the ethmoid cartilage (ec’eth. s.) has no longer any trace of the 
projection representing the anterior portion of the left supraorbital bar. 
The posterior portion of the bar (rd. sword. s. p.) projects forward from 
