RECAPITULATION 209 



the parent. The factors in the gametes of the 

 normal Flat-fish egg cause the normal metamorphosis 

 to take place after the larval symmetry has lasted a 

 certain time. In occasional individuals the factors 

 whatever they are, portions of the chromosomes or 

 arrangement of the chromosomes or anything else, 

 are different from those of the normal egg, and in 

 consequence the abnormalities above described are 

 developed. But the chief fact which I cannot too 

 strongly emphasise is that the development of the 

 abnormality from the symmetrical larva is direct, 

 whether it is merely an arrest of development or an 

 abnormal combination of reversed and normal parts. 

 The abnormal development is not due to a change 

 occurring after the normal asymmetry has been 

 developed. These abnormalities are true mutations. 

 The evolution of the normal Flat-fish, on the other 

 hand, was obviously due to a change of a different 

 kind. Here we are dealing with the change from 

 a symmetrical fish to the asymmetrical. Judging 

 from what takes place in other mutations, it was 

 quite possible for asymmetry to have developed 

 directly from the egg, in consequence of some 

 difference in the chromosomes of the nucleus. It 

 has been shown that placing a fish egg for a short 

 time in MgCl 2 * causes a cyclopean monstrosity to be 

 developed in which the two eyes are united into one : 

 but the two eyes do not develop separately first and 

 then gradually approach each other and unite, the 

 development of the optic cups is different from the 

 first. In the normal Flat-fish the evolution that has 

 occurred is the original development of the sym- 

 metrical fish, and the subsequent continuous gradual 



1 Stockard, Arch. Eut. Mech., xxiii. (1907). 

 O 



