LOSS OF PAIRING SEGMENT 375 



of mother-cells in Hitmulus japonicus (according to Sinoto). The 

 non-disjunction is due to the same cause, irregular orientation of 

 the chain at metaphase, and has the same effect, the formation of 

 gametes with too many and too few chromosomes, as non-dis- 

 junction in other cases of diploid ring and chain formation in inter- 

 change heterozygotes {cf. Sinoto, 1929 ; Winge, 1929 ; Kihara, 

 1929 b). 



Two kinds of segregation of sex chromosomes do not follow 

 prophase pairing and chiasma formation. The first of these is 

 found in certain Hemiptera [Lygceus, Wilson, 1905 a) where the 

 X and Y chromosomes are unpaired at the first division but regularly 

 lie on the equator and divide when the bivalents separate. Their 

 halves then pair, end-to-end, at the second division and pass to 

 opposite poles. This pairing is perhaps due to a property analogous 

 to terminal affinity (cf. Ch. XII). 



The second has been described as " distance conjugation." It is 

 found in Neuroptera (Naville and de Beaumont, 1933 ; Klingstedt, 

 1933) and Hepaticae (Lorbeer, 1934). In the Neuroptera the X and 

 y chromosomes, which have been lying on opposite sides of the 

 diakinesis nucleus, pass to opposite ends of the spindle at metaphase. 

 In the Hepaticae, on the other hand, the distant conjugation 

 described is spurious, being merely the result of one pair of chromo- 

 somes, usually the sex chromosomes or the smallest pair, having 

 terminal chiasmata owing to their short pairing segments, while the 

 rest have interstitial chiasmata {cf. Secale, Fig. 49). 



Where pairing is by terminal affinity a minute pairing segment 

 is presumably left. Where segregation follows distant conjugation 

 there is no longer any need for a pairing segment. Since unpaired 

 chromosomes in ordinary mutants and hybrids segregate irregularly, 

 the regular segregation of unpaired chromosomes like the X in the 

 Orthoptera and X and Y with distance conjugation in the Neu- 

 roptera requires a special explanation. The clue for this explanation 

 is provided by three particular observations. The sex chromosome 

 pair in the mammals has a weaker staining reaction than the 

 autosomes at metaphase, and usually lies at the edge of the plate 

 or to one side of it. It presumably has a lower surface charge 

 (Koller and D., 1934). When the two are unpaired they can be 



