Basis of Sex Determination 199 



of each male possessed a nucleolus, while in the other 

 half it was lacking. Montgomery afterward showed 

 that Henking's nucleolus was an accessory chromosome. 

 McClung was the first to recognize the importance of 

 this fact for the problem of sex determination. He 

 observed an accessory chromosome in one-half of the 

 spermatozoa of two forms of Orthoptera, Brachystola 

 and Hippiscus, and reached the following conclusion: 



A most significant fact, and one upon which almost all 

 investigators are united in opinion, is that the element is 

 apportioned to but one-half of the spermatozoa. Assuming 

 it to be true that the chromatin is the important part of the 

 cell in the matter of heredity, then it follows that we have 

 two kinds of spermatozoa that differ from each other in a 

 vital matter. We expect, therefore, to find in the offspring 

 two sorts of individuals in approximately equal numbers, 

 under normal conditions, that exhibit marked differences 

 in structure. A careful consideration will suggest that 

 nothing but sexual characters thus divides the members 

 of a species into two well-defined groups, and we are logi- 

 cally forced to the conclusion that the peculiar chromosome 

 has some bearing upon the arrangement. 



N. M. Stevens and E. B. Wilson 1 have not only 

 proved the correctness of this idea for a number of 

 animals but have laid the foundation of our present 

 knowledge of the subject. Wilson shewed that in those 

 cases where there are two types of spermatozoa, one 



1 Wilson, E. B., "Studies on Chromosomes, " Jour. Expcr. Zool., 1905, 

 ii-i 371, 57; J90 6 . iii-. i; 1909. vi. t 6 9. *47; I9 IO > ., 53; 1912, xiii., 345. 

 "Croonian Lecture," 1914, Proc. Roy. Soc., B. Ixxxviii., 333. 



