XV] 



Moment of Segregation 



269 



to the operation of ferments their diversity must be anyhow 

 very great, and it seems strange that all these multifarious 

 potentialities should exhibit gametic allelomorphism. Let 

 us take an illustration. Colour, as we can prove in regard 

 to several plants and as we suspect in the case of animals, 

 is due to the meeting of two complementary factors. One 

 is presumably a ferment. Recent research strongly suggests 

 that it may be a tyrosinase^. The other is referred to 

 sometimes as " chromogen." But whatever they are, the 

 two bodies — and surely the factors which produce them — 

 must be of utterly different nature, and yet genetically the 

 two potentialities are treated similarly. Each is allelo- 

 morphic to the absence of such a power. 



Similarly in regard to the anthocyanin colours of 

 Antirrhinum Miss Wheldale has given reasons for the 

 belief that they are formed by the meeting of a tannin- 

 like body with a ferment, perhaps an oxydase. Both these 

 factors, whaitever be their nature, are allelomorphic to their 

 absences. 



How much more astounding Is it, that when we pass 

 to qualities such as length of stalk, and shape of flower, the 

 shape of a cock's comb, or the development of interlocking 

 barbules on the webs of its feathers t, we still find the same 

 rules in undeviating operation ! 



Moment of Segregation. 



At what particular cell-division of the many by which 

 the germinal cells are brought to their maturity does this 

 process of segregation happen .f* The question is of extreme 

 interest, but no positive answer can yet be made. Natu- 

 rally the common expectation we all share is that the 

 reduction-division is the critical moment. At that division 

 the number of chromosome-elements, of which the nucleus 

 is formed, can be seen to be halved. Up to this point the 

 nucleus of each daughter-cell seems to be simply a repe- 

 tition of the nuclei of the body and may be supposed to 

 contain all the elements which they contain. But when 

 the number of these elements is halved, the germ-cell 



* See F. M. Durham, Proc. Roy. Soc. 1904, vol. 74, p. 311. 

 t Distinguishing them from the loose, non-cohering webs of the feathers 

 in the Silky breed. 



