176 MR. J. T. CUNNIKGHAM OX [Feb. 19, 



It is true we might use the word reversal, but this is not suffi- 

 ciently distinct. In order to avoid confusion it will be better to 

 coin a new term, and it seems to me the most appropi-iate term is 

 " metastrophe,'* meaning a change in the direction of the turning. 

 For adjectives we may use merely sinistral or dextral, referring to 

 left or right side, or for the abnormal condition in general we may 

 use metastrophic. 



Since, then, metastrophe frequently occurs in jlat-fishes, and is 

 a congenital abnormality due to some abnormality in the constitu- 

 tion of the ovum, it is intelligible that it should occur in one part 

 of a fish and not in another. We may suppose the abnormality 

 in the whole fish is due to the interchange of position in the ovum 

 of the parts corresponding to the left and right sides of the body. 

 The abnormality does not, however, afiect the viscera, which, as I 

 have pointed out in the memoir already cited, ai-e constant in 

 position whethei- the fish is dextral or sinistral. In the particular 

 specimen of Turbot which we are considering, the head is dextral, 

 or metastrophic, the posterior portion normally sinistral, and its 

 origin is to be attributed to a corresponding abnormality in the 

 constitution of the ovum from which the fish was developed. 



With regard to the question of the origin of such abnormalities 

 in the ovum, they may arise either in the cell-divisions which 

 occur in the multiplication of ova or spermatozoa of gametes, 

 to iise the general term, or in the process of fei^tilisation, the 

 conjugation of the gametes. It might be suggested in this 

 particular case that the condition was due to a " cross " between 

 an abnormal dextral specimen and a normal sinistral specimen, 

 the condition of the head-region being inherited from one parent 

 and that of the posterior region from another. But metastrophic 

 or dextral specimens are, so far as my experience goes, rare in the 

 Turbot, and it seems equally possible that the peculiar condition 

 of the gamete which gave I'ise to the abnormality was not due to 

 the condition of one of the parents. 



It is not necessary to suppose that both of the gametes which 

 produced the fertilised ovum were abnormal : abnormality in one 

 only may have been sufficient to produce the abnormality of 

 development. In the division of the gametes within the repro- 

 ductive organ of a parent fish, the chromosomes of the nucleus, 

 which are supposed to be the " carriers of heredity " or to contain 

 the "determinants" which produce the characters of the organism 

 to which the gamete gives rise, normally divide severally so 

 that two similar ova are produced. In the final or reduction 

 division each chromosome does not divide, but the group of 

 chromosomes separates into two groups. In one or other of 

 these divisions the determinants might be displaced, so that either 

 all or some of those belonging to the left side were on the right 

 and vice versa, and thus a metasti'ophic gamete would be produced. 



One important question that arises from the condition observed 

 in the specimen under discussion is, what bearing it has on the 

 expei'iments carried out by me some years ago at the Plymouth 



