1907.] AX ABNORMAL TURBOT. 179 



is metastrophic and the posterior region normal, as in the specimen 

 here described, or vice versa, as in ambicolorate specimens pre- 

 viously described, then the normal relation of the determinants of 

 these parts in the ovum, and therefore in development, is wanting. 

 The anterior end of the dorsal fin belongs to the posterior of the 

 two portions abnormally joined in the fish. It tends to grow 

 forward, but in the normal case in doing so unites with the I'iglit 

 side of the skull (in the Turbot) ; whereas in the abnormal specimen 

 here described, where the head is metastrophic, it lias the left side 

 of the skull opposite to it, and with this side it has no congenital 

 relations, and so remains separate from it. In the more usual 

 case, where the eyes are on the left side as usual but the fish is 

 ambicolorate, a similar explanation would apply. Here the right 

 side of the skull is opposite the fin, as in the normal fish ; but the 

 fin being itself metastrophic, the normal relations between fin 

 and skull in development are disturbed, and consequently they 

 remain separate. It may be said in fact that in all these cases 

 the fish, or the ovum from which it develops, is composed of two 

 separate parts united in an abnormal relation to one another in a 

 plane transverse to the long axis of the fish. Consequently the 

 normal continuity between the head and body is, as it were, 

 imperfect ; and in all probability this is the real reason why in 

 these cases the anterior end of the dorsal fin remains unattached 

 to the head. 



The abnormality of the dorsal fin does not occur in specimens 

 which are entirely metastrophic. Here, although the characters 

 of the right side develop on the left, and vice versa — that is to 

 say, the determinants of the right and left sides have changed 

 places — the dislocation of determinants in the gamete has taken 

 place along the median plane, and therefore the longitudinal 

 continuity between fin and skull is not disturbed. 



It is important to mention that the abnormality of the fin in 

 the specimen here described is not merely due to incomjjlete 

 metamorphosis. The normal specimen of the same size, or rather 

 smaller, sent with the abnormal, and captured at the same time, 

 shows complete metamorphosis, and in it the dorsal fin extends 

 forward attached to the head to a point anterior to the eyes. 



The correlation between ambicol oration and the abnormality of 



the dorsal fin is not invariable. Cases occur in which ambicolorate 



specimens are in this respect structural!}^ normal. In the Phil. 



Trans, memoir by myself and Dr. MacMunn (referred to above, 



p. 175), I made the generalisation from the specimens of Turbot 



then known to me, that if pigment was present over the whole of 



the body behind the pre-opercular bone, and also on the lower jaw 



and the anterior end of the dorsal fin, the malformation of the dorsal 



fin was present ; whereas if the pigment was less than this, the 



malformation was absent. On the hypothesis of the cause which 



I have suggested, the absence of the malformation in the latter case 



is intelligible, for then the junction between the metastrophic and 



the normal parts of the body may be supposed to occur not between 



1 o* 



