6 PROCEEDINGS OF THE AMERICAN ACADEMY 



embryo is seen in profile (see PI. IV. figs. 3-5, PI. VI. figs. 6, 7, PI. V. 

 figs. 8-12, PI. VII. fig. 5), until. the eye o-f the blind side has, for all 

 practical purposes, passed over to the colored side (PL V. figs. 4, 11). 



The rapidity and extent of this translation and rotation of the eye 

 from the blind to the colored side can be best seen on comparing the 

 profiles of the heads (PI. V. figs. 5, 10) of a dextral and a sinistral 

 Flounder with the profiles seen from the colored sides, before the eyes 

 have begun their movement (PL I., PL VL fig. 6, PL VIL fig. 5). 



As the dorsal, little by little, with advancing age, extends along the 

 head towards the nostrils, it soon, in old specimens, finds its way behind 

 the eye which has come from the blind side (compare the position of 

 the anterior part of the dorsal, in PL VI. figs. 5 and 7, in PL IV. figs. 

 2 and 5, and PL VIII. fig. 3). This continued advance of the dorsal 

 anteriorly, after the eye has passed to the colored side, naturally gave 

 rise to a great many theories respecting the passage of the eye through 

 the head, under the anterior part of the dorsal fin ; and many natu- 

 ralists, after an examination of the twisted facial part of the skull on 

 the adult, have attempted most ingenious explanations of the mode by 

 which the eye reached its ultimate position. 



The facts contained in this paper leave no doubt that, at any rate, in 

 the majority of the Flounders of our coast (I have traced the devel- 

 opment of eight species), the transfer of the eye from the blind side 

 to the color side occurs very early in life, while all the facial bones 

 of the skull are still cartilaginous, and that long before their ossifica- 

 tion the eye has been transferred, by a combined process of translation 

 and rotation, to the colored side. Let x^ y, z be rectangular axes ; 

 and, if we call the longitudinal axis of the body twisting x, the trans- 

 verse axis at the extremity of which the eyes are placed in the plane 

 xy^ the first change taking place is that x is no longer at right angles 

 with y, though the eyes are still in plane xy. The next change is 

 that the plane in which the eyes are now placed {x'y') makes an angle 

 with the xy ; cutting z at a, slight distance above the origin of the co- 

 ordinate axes, the eye of the colored side forming the apex of the angle. 

 This angle gradually increases, until it passes beyond the plane yz, 

 when the eye from the blind side has reached the colored side. 



The subsequent modifications of the frontal bone, owing to the 

 aberrant position of the eye from the colorless side, are interesting on 

 account of their connection with abnormal anatomical features found 

 in the Flounders ; but they explain in no wise the mode in which the 

 transfer of the eyes has taken place, this being anterior to any essen- 

 tial changes in the frontal bone. In early life, the strong muscles which 



