206 METAMORPHOSIS AND 



specimen to which I refer the pigmentation instead of 

 being present on both sides was reversed : the lower 

 side was pigmented from the posterior end to the edge 

 of the operculum (Plate n., fig. 2), while the upper side 

 was unpigmented excepting a scattering of minute 

 black specks and a little pigment on the head (Plate 

 n., fig. 1). I have suggested that the explanation 

 here is that in the zygote the primordia of a normal 

 body and a reversed head have been united together. 

 We may suppose that different parts of the body are 

 represented in the gametes by different determinants 

 or factors, and therefore it is possible that these 

 factors may be separated. In the specimen we are 

 considering the body is normal or nearly so, with 

 the pigmentation on the left side, which is normal 

 for the Turbot, while the head has both eyes with 

 some pigment on the right side and the left side 

 unpigmented. Reversed specimens occasionally 

 occur in many species of Pleuronectidae, and if the 

 determinants for a reversed head and a normal 

 body were united in one zygote, the curious 

 abnormality observed might be the result. It is 

 just a possibility that if this fish which was only 

 4 '4 cm. long had lived to adult size, the upper side 

 would have become pigmented under the influence 

 of light, while the strong hereditary influence would 

 have prevented the disappearance of the pigment 

 from the lower side. In that case the adult condition 

 would have been similar to that of ordinary ambi- 

 colorate specimens, but reversed, with eyes on the 

 right side instead of the left. Other explanations 

 of the more frequent ambicolorate mutation are 

 possible: the body may consist of two left sides 

 instead of a left and right, joined on to a normal 



