THE HEREDITY OF SEX 59 



veniently name it the sex-chromosome. It is often 

 distinguished by its larger size and different shape. 

 Wilson describes the following different cases : — 



(1) The sex-chromosome in the male gametocytes 

 is single and fails to divide with the others, but 

 passes undivided to one pole. This may occur in 

 the first reduction division (Orthoptera, Coleoptera, 

 Diptera) or in the second (many Hemiptera). But 

 it is difficult to understand what is meant by ' fails 

 to divide.' In one of the reduction divisions all the 

 chromosomes divide as in ordinary or homotypic 

 nucleus division, but in the other the chromosomes 

 simply separate into two equal grouj)s without 

 division. If there are an odd number of chromo- 

 somes, 2N—1, in all the gametocytes of the male, as 

 stated in most accounts of the subject, then if one 

 chromosome fails to divide in the homotypic division, 

 we shall have 2N~ 2 in one spermatocyte and 2N— 1 

 in the other. Then when the heterotypic division 

 takes place and the number of chromosomes is 

 halved, we shall have two spermatocytes with N— 1 

 chromosomes from one of the first spermatocytes 

 and one with N and one with N—l from the other. 

 Thus there will be three spermatozoa with N—l 

 chromosomes and one with N chromosomes, whereas 

 we are supposed to find equal numbers with N and 

 N—l chromosomes. It is evident that what Dr. 

 Wilson means is that the sex-chromosome is unpaired, 

 and that although it divides like the others in the 

 homotypic division, in the heterotypic division it has 

 no mate and so passes with half the number of 

 chromosomes to one pole of the division spindle, 

 while the other group of chromosomes has no sex- 

 chromosome. Examples of this are the genera 



