266 GENETICS 



This he called a sex-determiner. The association of this 

 chromatic body with sex determination proved to be 

 a discovery of primary importance. In fact it opened 

 a new era in cytology and heralded the beginning of 

 a large number of experiments and much profitable dis- 

 cussion dealing with the mechanism of sex determina- 

 tion. 



In many groups of animals there is an unpaired 

 chromosome in the male, called the ^-chromosome, 

 which may be seen in the somatic cells, in the sperma- 

 togonia and in the spermatocytes (Fig. 81). In the 

 female cells, both somatic and germinal, the #-chromo- 

 some is paired. During the process of spermatogenesis 

 the autosomes, that is, the remaining chromosomes 

 which have nothing to do with the determination of sex 

 pair to form tetrads, in the first spermatocyte division, 

 but are later reduced to dyads, in the second sperma- 

 tocyte division, when the ^-chromosome passes un- 

 divided to one of the second series of spermatocytes 

 (Fig. 81). The other spermatocyte of the second 

 division accordingly receives no part of the sex-deter- 

 mining material. During the division into spermatids 

 the former second spermatocyte, now freighted with 

 the .r-chromosome, gives rise to a?-bearing cells which 

 form the female-determining sperm, while the other 

 second spermatocyte, which did not receive an iT-chro- 

 mosome, gives rise to two male-determining spermato- 

 zoa. 



Hence, any zygote receiving two sets of autosomes 

 and two ^-chromosomes becomes a female, while a 

 zygote receiving two sets of autosomes and only one 



