VARIABILITY OF Y CHROMOSOME 363 



sex mechanism with that of the complex heterozygotes in (Enothera. 

 This second instance of the preservation of a demonstrable form of 

 structural hybridity by the sex heterozygote shows the essential 

 similarity between the complex and sex heterozygote. 



5. Even more complex conditions are found in Blaps lusitanica 

 (Nonidez, 192 1) and in various Hemiptera and Nematoda. In 

 these the X or Y chromosome is represented by several separate 

 bodies in somatic cells. They unite at meiosis, and pair with the 

 opposite sex chromosomes. It would seem that these are apparently 

 inert supernumerary chromosomes derived by fragmentation from 

 X or y and therefore continuing to pair with their homologues 

 (Table 16). 



6. The variation which occurs in the relative size of the X and Y 

 chromosomes might be supposed to have two extremes, the 

 complete disappearance of one or the other. But the X chromosome 

 never disappears in organisms with the XX — XY system. That 

 is to say, the distinction between the sexes never consists in the 

 presence of a single supernumerary chromosome in one sex which is 

 not represented in the other. The opposite extreme, in which the 

 Y chromosome has disappeared, is very common in animals, e.g., 

 Orthoptera, Hemiptera, Coleoptera and Nematoda. The sex 

 chromosomes are said to be of the XO type. The random segre- 

 gation of the single X chromosome gives two classes of gametes 

 with and without X. These can be distinguished in the living 

 sperm of the nematode Ancyr acanthus (Mulsow, 1912), and the 

 direct connection therefore established between segregation and 

 sex determination. The X chromosome (as in the XY type) may 

 have several independent components (e.g., Perla marginata and 

 various Neuroptera). These may fuse temporarily at meiosis [e.g., 

 Nematoda and Aphides, Morgan, 1912). In Perla they fuse at 

 diakinesis, but are independent at metaphase. They nevertheless 

 pass to the same pole. The two poles must therefore be in some 

 way asymmetrical in their development. 



7. This last type may give a secondary XY mechanism owing to 

 the fusion of the X chromosome with an autosome, giving an 

 unequal pair at meiosis (de Sinety, 1901 ; McClung, 1905, 1914, 

 1917 in Orthoptera). The origin of this type is betrayed by its 



