REPRODUCTION AND HEREDITY 127 



female is characterised by an even number of chromosomes 

 («) and the male by an odd number, one less (w — i). All 



the mature eggs have - chromosomes, while the ripe sperms 



have either - or - — i , the latter being clearly male, and the 



former female- determining. At the pairing of chromo- 

 somes for the maturation-process in males of such insects 



tCMM* 



••.. 



fill ••» 



• MMM V 



Fig. 33. — Diagram of Chromosomes of Bug {Lygaem). a, Diploid 

 group of male insect in pairs, one " sex-chromosome " (.r) being much 

 larger than the other {y). d. Diploid group of female, no such differ- 

 ence apparent, c, Maturation division in sperm formation .v and v 

 separating to the two daughter-groups shown in polar \-iew at t/, " male- 

 producing " sperm with y, "female-producing" ^^•ith x. After E. B. 

 Wilson {Journ. Exper. Zool. ii, 1905). 



there is a sex-chromosome which has no partner ; there- 

 fore in the reducing division half the sperms will receive a 

 chromosome more than the other half (Fig. 32, ^). In such 

 cases where the male- determining sperm is distinguished by 

 the absence of a sex-chromosome, it seems inappropriate to 

 state that the male-factor is *' dominant," though the result 

 is comparable to dominance in ordinar}' Mendelian 

 inheritance. 



It is also of great interest to know that in other insects 



