192 INDIVIDUALITY OF CHROMOSOMES [CH. 



A similar and still more striking instance of the same sort 

 of thing will be described in a later chapter when discussing 

 "non-disjunction" of chromosomes in Drosophila. 



Other examples of abnormal behaviour of chromosomes 

 causing a departure from the typical number, but support- 

 ing the individuality hypothesis are provided by the obser- 

 vations of MONTGOMERY on Euschistus (1910) and of 

 WILSON on Metapodius (1909), two genera of Hemiptera. In 

 Euschistus MONTGOMERY found that not very rarely a pair 

 of chromosomes failed to conjugate in the first spermatocyte 

 division, but remained separate and divided equationally. 

 In nearly ah 1 cases, however, they corrected this deviation 

 from the normal behaviour by conjugating and separating 

 from one another in the second division. Still more striking 

 are WILSON'S observations on Metapodius; already referred 

 to in Chapter XI (PL XVIII). He finds that in different 

 individuals there is a variable number of "supernumerary" 

 chromosomes, which behave in the spermatocyte divisions 

 in the same way as idio-chromosomes. Although the number 

 varies in different specimens, it is constant in the cells of 

 any one specimen, and all the various numbers found can 

 be accounted for on the assumption of the occasional passing 

 of the 2^-chromosome to the same pole as the X, giving 

 rise to a spermatozoon with an extra chromosome, if it is 

 conceded that this chromosome retains its individuality. 

 Furthermore, there is in Metapodius one pair of very small 

 chromosomes, and in all the sperm-cells of one specimen 

 WILSON found that instead of a normal supernumerary 

 there was an extra chromosome which in the first spermato- 

 cytes united with this pair and passed with one of them to 

 the pole. In appearance it was indistinguishable from an 

 ordinary supernumerary, but its different behaviour proved 

 its individuality. 



