WILLIAMS: MIGKATION OF EYE IN PSEUD0PLEUR0NECTE3. 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 Pai'ker'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 

 ever the mid-braiu. In front, the skull is roofed over with a thin carti- 

 laginous plate, the ethmoidal " tentorium," or tegmen cranii. Anteriorly 

 this is directly 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 trabeculse 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 liave their 

 origin on the posterior face of the ethmoid. The recti originate from a 

 lamina on tlie 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 Bern'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 

 in Figure 7 (Plate 1) as they appear in frontal projection. As in the 

 salmon (Parker, '73), the first cartilages to form are the trabeculee 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 trabeculee, dividing the mass at the anterior end, which is to be 



