398 l. doncastkr 



Further Observations on Chromosomes and Sex- 

 determination IN Abraxas pjrossulariata. 



In a previous paper (' Journ. of Genetics', iv, 1914, p. 1) 

 I described the inheritance of a tendency to produce families 

 consisting chiefly or entirely of females in Abraxas g r o s s u- 

 lariat a , and attempted to correlate it with the behaviour of 

 the chromosomes. It was found that females of the strain in 

 which unisexual families occurred have fifty-five chromosomes 

 as the somatic number, while all males and most other females 

 have fifty-six. In the maturation of the fifty-five-chromosome 

 strain, twenty-eight chromosomes travel to one pole of the 

 first polar spindle and twenty-seven to the other. Since all 

 spermatozoa were found to have twenty-eight, it seemed 

 evident that eggs with twenty-seven must be female-determin- 

 ing, since the union of an egg having twenty-seven with a sperm 

 having twenty-eight would give fifty-five, the number found in 

 females of the strain in question, while eggs with twenty-eight 

 meeting sperms with twenty-eight would give the fifty-six found 

 in the male. Evidence was also given that in families in which 

 great excess of females was produced, a majority of the eggs 

 matured in such a way that twenty-eight chromosomes were 

 extruded in the first polar nucleus, and twenty-seven remained 

 in the egg-nucleus, and it was therefore inferred that the 

 condition in some families, in which only females were produced, 

 was caused by the invariable extrusion of the twenty-eighth 

 chromosome in the polar body, leaving all eggs with only 

 twenty-seven, and therefore female-producing. This hypothesis 

 was supported by the observations of Morgan on Phylloxera, 

 in which one chromosome is always extruded in the polar body 

 of male-producing eggs, although it is already determined in 

 some other way that these eggs will become males. 



When the paper referred to was published, I had been able 

 to obtain no completely conclusive evidence that in families 

 consisting wholly of females all the eggs had only twenty-seven 

 chromosomes in the egg-nucleus, and I spent the next two 

 seasons in collecting material which it was hoped would give 



