58 MINOR ABNORMALITIES AMBICOLORATION 



an exception) the migration of the eye from the blind side takes place prior to the forward extension 

 of the dorsal fin. Speaking in mechanical terms, one may say that marked incompleteness of migra- 

 tion is able to interpose a direct obstacle in the way of this extension, e.g. in the " cyclopean " types. 

 On the other hand, simple delay in the migration may hold back the extension in question till the 

 fin- and cranial-tissues have lost their early powers of mutual union and growth. The delayed 

 migration of the eye may in the end be fully carried out, e.g. in the instances mentioned under (2) 

 above, or it may remain somewhat incomplete, as in the numerous instances where the eyes are set 

 slightly further apart than in the normal condition. 



Probably, however, the arrested migration of the eye is significant, less from actually 

 obstructing the forward growth of the dorsal fin, than in being an expression of a general aberration 

 of development involving other departures from the normal moulding of the head, some of which fall 

 under the category of excess, and others under that of defect. As Traquair (BSla) pointed out, the 

 dorsal fin of a flat-fish does not extend forward in the morphological middle line of the head, but is 

 distinctly to the blind side of this line. One of the defects may well be that a new mid dorsal edge 

 of the head is never prepared at all or never twisted round sufficiently to enable it to unite with 

 the dorsal fin. 



Among characters indicating excess, we may place the undue development of spines, tubercles, 

 etc., on the under side. This can occur without pigmentation of the side in question. As Bateson 

 points out, it must be put down in the main not to reversion or atavism, but to the principle of 

 Homoeosis, i.e. to the tendency towards secondary or " imitative " bilateral symmetry. 



An interesting attempt at analysis of the germinal factors involved has been made by Cunning- 

 ham in connection with his description of the brill (R. laevis), to which attention was previously 

 drawn. He suggests that the determinant system of the ovum may have been altered in such a way 

 as to match a reversed flatfish type of body, with a normal flatfish type of head, the resulting 

 misfit being evidenced by the condition of the dorsal fin. 



This might account for the presence of pigment on the under side of the body and its absence 

 from the under side of the head. The upper side of the body, though white at first, would darken 

 later, since, as Cunningham had previously shown (50a), pigmentation can be induced even on the 

 under side of a flat-fish, through exposure to light. The theory fails when applied to ambicoloration 

 in the flounder, since specimens not infrequently occur in which the under side of the head is 

 pigmented as well as the under side of the body. Again, as Cunningham himself points out, 

 it does not explain the presence of the characteristic spines and tubercles which occur on the upper 

 side of ambicoloured as of normal flat-fish. The arrested migration of the eye is also not 

 satisfactorily accounted for, and on the whole the explanation besides depending on theoretical 

 conditions does not carry us as far as the facts themselves. 



As Cunningham rightly emphasised, the point which needs explanation is that in ambicoloured 

 flat-fish the under side of the head is usually either unpigmented or has less pigment than the rest of 

 the under side. One might suggest the following consideration as supplying a not too far-fetched 

 reason. Whiteness of the under side is not merely a reduction phenomenon, but is of importance to 

 fishes in general by rendering them less conspicuous in the water, and thus enabling them more 

 readily to obtain food or escape from enemies. This does not apply in the ordinary way to flat-fish, 

 but even to these it is likely to be of some importance in feeding, 1 that the head as seen from below 

 should not appear so conspicuously black as the presence of pigment, together with the want of 

 incident light, would make it. Whiteness of the under side of the head (and in a less degree of the 

 parts adjacent) might thus come to be a character of positive importance, and accordingly less liable 

 to variation, than the condition as regards colour of the rest of the under surface, a condition which 

 is traceable more directly to simple reduction. Even in Cunningham's own experiments, it was only 

 slowly and with difficulty that pigment could be induced to appear on the blind side of the head. 



That in most cases the origin of the complex of abnormalities under discussion is a germinal 



1 Crustacea and Polychaeta seem to be staple articles of diet with certain flat-fish, e.g. the plaice (Masterman in Vol. XIII. 

 Des Rapports et Proc&s- Verbaux du Council International pour I' Exploration de la Mer, Jan. 1911, p. 17). 



