WILLIAMS : MIGEATION 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 {trb. 

 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 is no 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 supraoi'bitals are connected, there is a median- 

 elevation in the sagittal plane of the fish {ms'eth., Fig. A), and a forward 

 knob-like projection {crt. 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 {i'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 its 

 origin, and in some cases the superior oblique muscle of the right eye 

 has its origin also close to that of the feft eye, therefore at the left of the 

 sagittal plane. 



c. Stage HI a. 



Figure B is photographed from the model of the cartilages of a fish of 

 Stage HI. (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 repi-esenting the anterior portion of the left supraorbital bar. 

 The posterior portion of the bar (trb. su'orb. s. p. ) projects forward from 



