1907.] AX ABNORMAL TURBOT. 177 



Laboratory, and described in the joaper already cited. In those 

 experiments pigment was developed on the lower sides of Flounders 

 as a result of the incidence of light. Here we have a specimen 

 of Turbot in which the upper side is exposed to light and is not 

 pigmented, while the low^er side is pigmented. But it must be 

 noted that no adult specimen has been observed in which this 

 condition occurs. According to Miss Fox's letter quoted above, 

 the upper side of this young turbot had already acquired some 

 pigment during the two months in which it lived in her possession. 

 It is quite possible therefore that if the specimen had lived to 

 become adult, the upj)er or i-ight side would have become fully 

 pigmented in consequence of the action of light, and then the 

 specimen would have been exactly similar to other ambicoloi'ate 

 specimens of Turbot, except that it was metastrophic, the eyes 

 being on the right side instead of the left. 



In my experiments, I showed that when young fish in process 

 of metamorphosis were placed in the apparatus so that light fell 

 on the lower side and not on the upper, the normal hei-editary 

 changes were not arrested, pigment disappeared from the lower 

 side as under normal conditions, and it was only later, after 

 long exposure to light, that pigment was develoj)ed on the lower 

 side. Thus, as the specimen we are here considering had not long 

 passed its metamorphosis, there is nothing inconsistent with my 

 results in the absence of pigment from the right side, although that 

 side is uppermost and had been exposed to light for a short time. 

 The condition of the specimen here described suggests that the 

 usual ambicolorate abnormality is due also to partial metastrophe, 

 but that in these cases the antet^ior part of the body is normal or 

 sinistral, and the posterior part dextral. This view would explain 

 the remarkable fact, of which hitherto no explanation has been 

 given, that in the great majority of ambicolorate Turbot the lower 

 or right side of the head is urqoigmented, just as in the specimen 

 here described the left side of the head is unpigmented. The 

 limits of the pigmentation are not absolutely constant. In the 

 majority of specimens which I have seen, the pigmentation 

 extends on to the lower jaw and the anterior end of the dorsal fin, 

 while the rest of the head in front of the preopercular bone is 

 unpigmented. One specimen in my list, however, had pigmenta- 

 tion over the whole of the lower side, including the head. If the 

 explanation suggested is correct, it follows that the young of an 

 ambicolorate specimen immediately after metamorphosis is without 

 pigment on the postcephalic portion of the upper or left side, and 

 that it becomes ambicolorate in adult life in consequence of the 

 development of pigment on that sitle under the influence of light. 

 There is at present no direct evidence of this beyond the occurrence 

 of the specimen described in this paper, and the question must be 

 fui'ther investigated by the examination of large numbers of 

 young specimens. When pigmentation extends over the whole of 

 the lower side, including the head, it cannot be said that the head 

 ■of the fish is normally asymmetrical ; therefore the theoi-v of 

 Proc. Zool. See— 1907, No. XII. 12 



