xi] IDIO-CHROMOSOMES 161 



giving rise to a spermatozoon with both X and T. This, 

 fertilising an Jf-bearing egg, would give rise to a female with 

 two JT-chromosomes and a supernumerary (?*), and all the 

 observed combinations can easily be derived from this by 

 suitable matings. 



Another point that must be noted is that the X-chromo- 

 some (heterotropic chromosome) may be closely associated 

 with one of the ordinary " autosomes " ; this has been ob- 

 served as a constant condition in certain Orthoptera, and is 



I 



a 



A 



X X 



FIG. 1 8. Compound groups formed by union of the .ST-chromosome with other 

 chromosomes in Orthoptera (from WILSON, 1911). a, b after SINETY, 

 c h after McCLUNG. 



also sometimes found in the Nematode Ascaris megalo- 

 cephala, though in other individuals of the same species 

 the .^-chromosome is quite independent of the others 

 (Text-fig. 1 8). 



Although, therefore, the details vary considerably, it is 

 clear that in a large number of animals of very different 

 groups the male differs from the female in not having one, 

 or in some cases more than one, chromosome which is 

 present in the female; either the chromosome is absent 

 altogether in the male, or it is represented by an element 

 D. c. ii 



