WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 15 
4, CHANGES IN THE CARTILAGINOUS SKULL. 
In order to have freshly in mind the normal condition of the cartilagi- 
nous skull in fishes with which to compare the youngest flounder skulls, I 
give a brief statement of the essential parts of Parker’s (’73) paper on 
the skull of the salmon: 
In a salmon of the second week, according to Parker, the cartilaginous 
skeleton is fully formed. There is a large fossa on the top of the head 
over the mid-brain. In front, the skull is roofed over with a thin carti- 
laginous plate, the ethmoidal “ tentorium,”’ or tegmencranii. Anteriorly 
this is directiy continuous with the ethmoid; its posterior lateral cor- 
ners are connected with the cartilage of the auditory region by the supra- 
orbital bars, which curve upward and outward. The ethmoid is contin- 
uous with the trabecule cranii,—now fused together in front, but 
diverging behind,— which run backward forming a partial floor to the 
skull cavity. The superior and inferior oblique eye muscles have their 
origin on the posterior face of the ethmoid. The recti originate from a 
lamina on the hinder part of the parasphenoid. 
I have projected upon the frontal plane the cartilages of the facial 
region of Pseudopleuronectes in each of the four stages. But because 
of the great length of the dorso-ventral axis of the older stages, this 
method needs to be supplemented either by projections upon the sagit- 
tal plane or by some other process. The most satisfactory recon- 
struction is, of course, the model. Accordingly with the aid of sections 
I have modelled in wax by Born’s method the facial region of Stages 
II., III., and IV.,and cuts made from photographs of these models are 
given in the text. 
a. Stage I. 
A dorsal view of the cartilages of the facial region in Stage I. is shown 
m Figure 7 (Plate 1) as they appear in frontal projection. As in the 
salmon (Parker, ’73), the first cartilages to form are the trabecule cranii 
and Meckel’s cartilage. The slight want of uniformity in the shape of 
Meckel’s cartilage on the two sides may be merely an individual varia- 
tion. Certainly this cartilage is essentially symmetrical. The line 
passing through the middle (third) brain ventricle and between the 
lobes of the tectum and cerebrum I have assumed to lie in the sagittal 
plane in a normal fish of this stage. This plane, represented in projec- 
tion in the figure by the two ends of a fine line, cuts lengthwise the 
fused trabecule, dividing the mass at the anterior end, which is to be 
