208 METAMORPHOSIS AND 



parts. In a sinistral Flat-fish, whether it is normally 

 sinistral like the Turbot or abnormally like a ' re- 

 versed ' Flounder, the viscera are in the same position 

 as in a dextral specimen : the liver is on the left side, 

 the coils of the intestine on the right. Thus in a 

 reversed or sinistral Flounder, which is normally 

 dextral, the left side which is uppermost is still the 

 left side, but it has colour and two eyes, whereas in 

 the normal specimen the right side has these char- 

 acters and not the left. Thus we are forced to 

 conceive of the determinants in the chromosomes of 

 the fertilised ovum which correspond to the two sides 

 of the body, as entirely distinct from the deter- 

 minants which cause the condition or ' characters ' 

 of the two sides, unless indeed we suppose that 

 determinants of right side with eyes and colour occur 

 in some gametes and of right side without eyes and 

 colour in others, and vice versa, and that homozygous 

 and heterozygous combinations occur in fertilisation. 

 On this last hypothesis the mutation here con- 

 sidered might be a heterozygous specimen, with the 

 dextral condition dominant in the head and the 

 sinistral in the body. Or it might be somehow due 

 to what Morgan and his colleagues have called 

 crossing over in the segregation of heterozygous 

 chromosomes, so that a part corresponding to a 

 sinistral body is united with a part corresponding to 

 a dextral head. 



My conclusion from the evidence is that any 

 process of congenital development may in particular 

 zygotes exhibit a mutation, a departure from the 

 normal. We need not use the term heredity at all, 

 or if we do, must remember that in the present 

 argument it does not refer to any transmission from 



