Sex Dclcrminalioi 191 



male and the other two female. This beliaNior certainly 

 suggests an even separation of the sexes at the reduction 

 division, such as would be hrouf^ht about by the sex 

 chromosome mechanism. 



More recently Allen (i), after repeating' and con- 

 firming the foregoing experiment, made a systematic 

 cytological search for the X chromosome in Sphacro- 

 carpus. He now reports that one large chromosome 

 (X), exceeding in length and thickness the other chro- 

 mosomes, characterizes the cells of the female game- 

 tophyte, while the cells of the male gametophyte are 

 characterized by one very small chromosome (!'). His 

 investigation shows that in spore formation two of the 

 spores of the tetrad receive the large chromosome, while 

 the other two receive the small chromosome. 



This rather clearly establishes a sex chromosome 

 mechanism, but the situation is distinctly dilTerent from 

 that in animals. The sex chromosome mechanism in 

 animals provides for a differentiation of sexual individ- 

 uals in the diploid generation, the female being A'A' 

 and the male XY . The sexual individuals in Spluicro- 

 carpus, however, are of the haploid gametophyte gen- 

 eration, the female gametophyte regularly being A', the 

 male F, and the sexless diploid sporophyte generation 

 being regularly A'F. In the animal mechanism, sex is 

 really estabhshed only at the time of fertiHzation, while 

 in Sphaerocarpus it is established immetliately at the 

 reduction division. 



Marchal (16) has done an interesting bit of work which 

 further provides indirect evidence on a sex chromosome mechanism 

 for sex determination in the gametophyte generation. I'utiaria 

 is a ''dioecious" moss, and hence it may be assumed that the sexes 



