80 THE CHROMOSOMES 



both chromosomes and extends as far as one end 

 of the chromosome. The remaining parts of the 

 two chromosomes are 'not homologous. Thus each 

 sex- chromosome consists of two segments, a pairing 

 ■segment and a differential segment (Fig. 8). The two 

 pairing segments come together at zygotene and chias- 

 mata are later formed between them on either side 

 of the spindle attachments or on both sides. An X Y 

 bivalent is thus formed. At the first meiotic division 

 the differential segments of the X and Y go to 

 opposite poles and at the second division they divide 

 in the ordinary way. The sex -determining mechan- 

 ism in man is probably of this type but the details 

 have not yet been worked out. In some Marsupials 

 the sex-chromosomes are essentially of the same 

 kind as in the Rat, but the spindle attachments 

 appear to lie in the differential region. ^^ 



In Drosophila melanogaster we again have an XY 

 bivalent formed in the male ; the pairing segment 

 includes the spindle attachment and is genetically 

 inert. The X- chromosome has a single differential 

 segment which includes the spindle attachment 

 and contains all the * sex-linked ' genes except 

 bobbed, while the Y has two differential segments, 

 one on either side of the pairing segment. 



In some Bugs (Rhynchota) no pairing of the X- 

 and Y-chromosomes takes place at the first division. 

 This may be due to there being no common homo- 

 logous pairing region, or it may mean that pairing 

 is prevented at zygotene by positive heteropycnosis. 

 In LygaeuSj at any rate,^^^ the latter explanation 

 seems to be the more probable one. Both the X 

 and the Y divide at the first meiotic division so that 

 the two secondary spermatocytes which are formed 

 have the same chromosome set. At the second 

 meiotic division the X- and Y-chromosomes come 

 into extremely close approximation ; it is probable 

 that this is a delayed zygotene pairing of short 

 homologous regions which is rendered possible by 



