210 THE THEORY OF THE GENE 



lighter color, like the father; the sons a darker color, like 

 the mother (Fig. 121). The daughter gets a single Z from 

 her father ; the son also gets this Z from his father, but 

 another from his mother. This maternal Z carries the 

 gene for darker color that is dominant, hence the darker 

 color of her sons. 



In the silkworm moth, Tanaka has found a sex-linked 

 character, translucent skin of the larva, that is inherited 

 as though carried in the Z-chromosomes. 



In the moth Fumea casta there are 61 chromosomes in 

 the female and 62 in the male. After conjugation of the 

 chromosomes in the egg there are 31 chromosomes pres- 

 ent (Fig. 122a). At the first polar division, when the first 

 polar body is given off, 30 of the chromosomes (bivalents) 

 divide and pass to opposite poles ; the thirty-first sin- 

 gle chromosome passes undivided to one or the other 

 pole (Fig. 119b and b'). Half of the eggs will come to 

 contain 31, half 30 chromosomes. At the next polar divi- 

 sion all the chromosomes present divide, leaving each 

 egg with the number it had before this division (i.e., 31 

 or 30). In the, ripening of the sperm of this moth, 31 

 bivalent chromosomes are present after conjugation of 

 the chromosomes. At the first division the members of 

 each pair separate and at the second each divides. Each 

 spermatozoon carries 31 chromosomes. Fertilization of 

 the eggs gives the following combinations : 



Females = 61<^ 



30s 

 x 31 v 



Males = 62< 





In another moth, Talaeporia tubulosa, Seiler finds 59 

 chromosomes in the female and 60 in the male. In Soleno- 



