134 SEX INHERITANCE 



in the cells of the sporophyte generation (diploid). 

 In the female gametophyte (haploid) the larger 

 member of the pair is present, and in the male 

 gametophyte (haploid) its smaller mate. At the 

 reduction division in the sporophyte, when four 

 spores are formed, two of the spores in each tetrad 

 contain each a large chromosome and two a small 

 one. It is fairly well determined that from each 

 set of four spores, two give rise to female plants of 

 the gametophyte generation, and two to male 

 plants. In this case the males and females are 

 plants with the haploid number of chromosomes, 

 one with the large chromosome and the other with 

 the small one. The sporophyte being diploid con- 

 tains both chromosomes, which, as stated, are 

 separated again when the spores are formed, i.e., 

 at a time corresponding, it would seem, to the 

 maturation division of the egg and the sperm. 



It may be recalled in this connection that the 

 male bee and the male Hydatina are haploid organ- 

 isms; but in them, unlike the haploid gametophyte 

 generation of the liverworts and mosses, modified 

 maturation divisions occur. By means of altera- 

 tions in the maturation divisions female-producing 

 gametes only are produced. 



Sex-ratios 



Both the XX-XY and the WZ-ZZ types give 

 equal numbers of males and females, provided that 

 both kinds of gametes are equally viable, that 



