PAIRING OF X AND Y 373 



(a) In the Orthoptera, the unpaired X chromosome (which is 

 precocious during prophase) Ues to one side of the equator and 

 passes, without division, before the paired chromosomes to the 

 nearer pole. It then divides at the second division. 



(b) In the male Aphides (Morgan, 1912, 1915 ; Schwartz, 1932 ; 

 Suomalainen, 1933) and Nematoda (Mulsow, 1912) it lags behind 

 the bivalents, but otherwise behaves in the same way. 



(c) In some Hemiptera (Vanduzea arcuata, and species of Ceresa, 

 Boring, 1907), the unpaired A" chromosome may pass to the pole 

 either before or after the autosomes. 



{d) An unpaired X chromosome is known to divide equationally 

 at the first division in a few species and be included in two telophase 



^^C^^ ^i^^^ ^ali^^ ^^Oc^^ <7^af^\> ^^ 0^^^ 

 ^^P^ <^V0^ <^^9PP ^<i«i^^ ^^^ii(? "^^ii^CP 



A B 



Stenoiolhrus . Aphis P/idinus fllelinJrium Ly^Beus TenoJera 



Sphaetocarpus Rumex 



marii/ animals 



xo ^ xy ^ x/T' 



Fig. 117. — Diagram showing the methods of segregation of sex chromosomes 

 at the first anaphase normally occurring in different species. 



nuclei at the second division [Photimis consangnineus and P. 

 pennsylv aniens, Coleoptera, Stevens, 1909 a ; Anasa tristis, Wilson, 

 1905 h ; Alydtis Hemiptera, Renter, 1930 ; Protenor, Schrader, 

 1935 '> various Odonata). 



We see that the behaviour of unpaired sex chromosomes without 

 partners does not differ in essentials from that of unpaired autosomes 

 in plant hybrids, although animal cytologists have not usually been 

 aware of this similarity. The distinction is that their behaviour 

 is more regular, more exactly adjusted, by a means we shall consider 

 later. The mechanism of the pairing of sex chromosomes with 

 partners by chiasmata, on the other hand, is less well adjusted 

 than that of the autosome bivalents. 



Since only a part, and often only a small part, of the X chromosome 



