EV'OLUTION OF ASYMMETRY 13 



supraorbital bar of the opposite side. This bar of the ocular side becomes twisted 

 over towards that side of the head by the movement of the two eyes into their final 

 position, and the ethmoid region of the chondrocranium is likewise affected at the 

 same time. The supra-orbital bar of the ocular side is subsequently reabsorbed also, 

 except for its anterior part, which remains as a stump — the hamulus ethmoideus. 

 As soon as the shifting of the eyes has been completed the frontal bones make their 

 appearance in the positions already indicated. 



In all four species of Flatfishes investigated there is, thus, during the ontogeny 

 a lengthy preparation for a comparatively short metamorphosis. In view of this, it 

 seems highly improbable that any twisting of the skull has been brought about by 

 the efforts made by the fish to see with the lower eye. As Regan (1910B, p. 485) has 

 pointed out, " it is wrong to say that the two eyes are on one side as the result of the 

 twisting of the orbital region of the skull, for the first step is the migration of one eye 

 into the territory of its frontal bone, causing resorption of cartilage in the larva, and 

 in the adult producing the effect that the orbital part of its frontal ossifies round it or 

 even entirely outside it. The displacement of the frontal of the lower eye has 

 enlarged the area of that of the upper eye ; but it seems wrong to speak of any part 

 of the latter bone as a new formation, least of all that part which has the same position 

 and the same relations {except to the eye) as it would have if the skull were 

 symmetrical ". The movement of both eyes into their final position on the side of 

 the head is accompanied by a certain degree of torsion of the orbital part of the 

 cranium, but this is certainly not caused by the migration of the eye.^ 



The recent work of Kyle (1921) on the asymmetry of the Heterosomata is very 

 difficult to follow, and I am unable properly to understand his views as to the causation 

 of the migration of the eye. His description of the development during metamor- 

 phosis of a subocular ligament below the eye of the blind side, which " forces the eye 

 to follow the deflected frontals to the other side ", is somewhat confused, and the 

 interpretation is not in accordance with other embryological work. According to the 

 view expressed by this author, the ossifications which subsequently appear in this 

 ligament to form the pseudomesial bar of Traquair represent new and special structures ; 

 at the same time, he hints at the homology of the pseudomedial bar with the subocular 

 shelf found in a number of normal Bony Fishes. 



The phylogenetic process by which the asymmetry of the eyes of the Flatfishes 

 has been acquired and estabUshed has been a matter of considerable controversy, and 

 cannot be discussed here. Leaving out of consideration the theory that the change 

 from the normal bilateral condition was originally brought about by a single mutation, 

 it seems fairly certain that this change must have occurred through gradual modifi- 

 cation. The two chief lines of argument, based respectively on the Darwinian 

 principle of natural selection and the Lamarckian theory of the inherited effects of 

 use and disuse, have been well summarised by Cunningham {1890, p. 51 ; 1892, 

 p. 193). It seems reasonable to suppose that a symmetrical fish which took to resting 

 on its side would try to make some use of the eye on the under side, and that the 

 change in the position of the eyes may have been initiated by the continual pressure 

 of the lower eye against the edge of the frontal bone.^ The pressure from the migrating 

 eye was probably the original cause of the resorption of the supra-orbital bar lying in 

 its path, an event which now takes place in the ontogeny before there is any sign of 

 movement on the part of the eye itself, as a preparation for the subsequent invasion 

 by the eye of the territory of the frontal bone of that side. 



The important work of Parker and Mayhoff on the optic chiasma in the Hetero- 

 somata will be considered in the section devoted to reversal (p. 28). It will be of 

 interest to mention here, however, that, apart from the optic nerves and the larger 



^ The view advanced by Rosenthal, and afterwards elaborated by Steenstrup (1S64), that the 

 eye of the blind side has penetrated through the tissues of the bead to the place it now occupies» 

 and has there formed for itself a new orbit, is one which is supported by no evidence, either 

 anatomical or embryological. 



^ See Regan (1926, p. 85). 



