xi] LEPIDOPTERA 169 



In all the cases mentioned hitherto the male differs from 

 the female in producing two kinds of germ-cells, while the 

 eggs are all alike, and the natural conclusion to be drawn is 

 that in species which show this condition the sex of the 

 individual developing from a fertilised egg depends upon 

 the spermatozoon. For reasons that will be considered in a 

 later chapter, however, it has for some years been known 

 that at least in the Lepidoptera and Birds there must be 

 male- and female-determining eggs, and it was suggested 

 that in these groups of animals it was possible that the 

 converse condition might be found that the female might 

 have an odd chromosome or an unlike pair of idio-chromo- 

 somes while the male had two equal sex-chromosomes. 

 For some time the search for cases of this kind was unsuc- 

 cessful, but in 1913 SEILER discovered one in the moth 

 Phragmatobia fuliginosa. In this species he found 56 

 chromosomes in each sex, 54 autosomes and two large idio- 

 chromosomes, but in the maturation divisions the idio- 

 chromosomes behave differently in the two sexes. In the 

 spermatocytes they pair and separate like the autosomes, 

 so that all the spermatids receive 28. In the anaphase of 

 the first polar division of the egg, however, the two idio- 

 chromosomes behave differently. In the equatorial plate 

 of this division they appear as a bivalent, as in the male, 

 but in the anaphase one of them divides into two unequal 

 parts, while the other, which goes to the opposite pole, re- 

 mains as a single body. At the end of the first polar spindle, 

 therefore, there are 28 chromosomes at one pole (27 auto- 

 somes and one large idio-chromosome, as in the male), and 

 29 (27 autosomes and a divided idio-chromosome) at the 

 other. In some eggs the divided chromosome travels to the 

 outer pole and enters the polar nucleus, in others it goes 

 to the inner pole and is included in the egg-nucleus. There 



