THE DETERMINATION OF SEX 635 



For example, Wilson reports that as long ago as 1891 Hen- 

 king " discovered that in the second spermatocyte division of 

 Pyrrhocoris one of the chromosomes passes undivided into one 

 of the daughter cells (spermatids), which receives twelve chro- 

 matin elements, while its sister receives but eleven" ; so that, 

 of the four resulting spermatozoa, two possess an additional 

 chromosome as compared with the other two. 1 



Other discoveries were reported, and Paulmier (1898, 1899), 

 working with Anasa in Wilson's laboratory, found that in the 

 first spermatocyte division eleven tetrads appeared, one of which 

 was " much smaller than the others " and seemed " to arise from 

 a single nucleolus-like body . . . and by a process differing con- 

 siderably from the others." He adds : " In the second (and last) 

 spermatocyte division the (ten) larger dyads divide to form 

 chromosomes in the usual manner ; the small dyad, however, 

 fails to divide, passing over bodily into one of the spermatids. 

 In this case, therefore, half the spermatids receive ten single 

 chromosomes, while the remainder receive in addition a small 

 dyad." 2 



The fact was gradually established that, at least in Hemiptera 

 and in certain other insects, one of the chromatin masses of the 

 male maturation cell differs from its fellows, and undergoes one 

 less division than they, so that, of the group of four spermatozoa 

 arising out of the double division of the spermatocyte, two will 

 possess an additional chromosome as compared with the other 

 two. This additional member has been variously named by dif- 

 ferent experimenters, the terms " accessory chromosome" and 

 " heterotropic chromosome" being the most common. 



Here is about where the matter rested till McClung (1902) 

 advanced the theory that the accessory chromosome is the sex- 

 determinant t assuming (erroneously as we now believe) that if 

 the ovum should be fertilized by one of the spermatozoa contain- 

 ing the accessory chromosome the offspring would then be 

 provided with the accessory and its sex would therefore be 

 male ; while if the fertilization should be by one of the sperma- 

 tozoa destitute of the accessory, the offspring would of necessity 

 be of the opposite sex. 



1 Wilson, The Cell, p. 271. 2 Ibid. p. 272. 



