DIFFERENTIAL AND PAIRING SEGMENTS 365 



appear as a derivative of a diploid system, for a new polyploid will 

 maintain itself sexually only if it is hermaphrodite ; and it 

 will be likely to breed true to hermaphroditism. There is nothing, 

 however, to prevent differentiation arising de novo in an established 

 polyploid, as it doubtless has arisen in Fragaria. 



(ii) The Differential Segment. The dissimilar X and Y chromo- 

 somes pair at meiosis in the heterozygous sex. They diverge from 



1 2 X -Y.-Y, 



^ ^ S 6 7 6 ^ ,„ 



))§ 5—0 j-)— o-Zjjj 



U X*2Y+Y. Ill X+Y, +2Y, 



Fig. 114. — Methods of metaphase pairing in Rumex aceiosa with 

 extra sex chromosomes of each of the three types, X, Y^ 

 (equal in size) and Yg (the smallest). The limits of association 

 define the homologies of the chromosomes and the extent of 

 their differential segments. (Yamamoto, 1934.) 



the autosomes in their behaviour at this time in varying degrees. 

 This divergence can be shown to be related to the development of 

 differential segments between which, as in (Enothera, pairing at 

 pachytene, crossing-over and chiasma-formation are suppressed. 



The evidence of differential segments is of three kinds. First 

 the X and Y chromosomes may differ in size. It is then clear that 

 the part of one which is in excess must be differential with respect 

 to the other. The difference between the two may depend, as in 



