EVOLUTION OF X AND Y 



369 



that the first division is reductional for the XY pair. This means, 

 as we have seen earHer, that the two chromatids with the differential 

 segment of X pass to the opposite pole from those with the differential 



/ 



A 3 



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Mafxy P/an/s 



C 



r^i* 1 1 1 1 1 1 ■■ N I M 



Habrobraco/y. 



Z 



iiiiiininiynn, iniillin O- 



^nrmnrma 



A/g'/o/i dr/um 



uiiiiiiir.iimillk 



azmmnnmza 



P^rao/na/^oJbiQ. 



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iillinni^ii^ 



A'/a/i\/r\a// a. 



Co/co/i/'ero. 



ZmO Uliiiil Q iiiiiimninim 



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4 = 



OrtAoptera 



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M ETAPH A S £ 



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'2 



Fig. 115.— Diagram showing by the pachytene relationships of the 

 sex heterozygote the development of the diiferential segments 

 oiX (above) and Y (below) leading to the disappearance of Y (4). 

 The metaphase associations of certain types are given and depend 

 on the position of the differential segments (black and white) in 

 .. relation to the centromere. 



segment of Y at the first division. Where, however, the pairing seg- 

 ment is proximal, crossing-over occurs in some cells between the 

 centromere and the differential segment with the same results as 

 have already been described in considering the general behaviour of 



