xi] LEPIDOPTERA 171 



and the other 27, and as each kind of equatorial plate may 

 be either the inner or the outer, there are evidently two 

 kinds of mature eggs, some with 28 chromosomes in the egg- 

 nucleus and others with 27. Since all spermatozoa appear 

 to have 28, two kinds of fertilised eggs must exist, some 

 with 56 and others with 55, corresponding with the somatic 

 numbers in the male and female of this strain 1 . 



Up to the present time no other certain example has 

 been found of females which produce two kinds of eggs 

 differing in their chromosome number, but the two known 

 examples are sufficient to make it clear that in the Lepi- 

 doptera at least this condition can exist 2 . If, therefore, we 

 are justified in regarding sex as being determined by the 

 presence or absence of a particular chromosome introduced 

 into the zygote by one of the conjugating gametes, we must 

 conclude that in a number of animals the presence or 

 absence of this chromosome depends on whether it is or is 

 not borne by the spermatozoon, but that in the Lepidoptera, 

 and possibly some other groups, the spermatozoa are all 

 alike, and that the differential chromosome is carried by the 

 eggs. 



1 Although it can hardly be doubted that the production of males and 

 females in the strain of Abraxas in which the females have 55 chromosomes is 

 usually dependent on the presence of either 28 or 27 in the egg-nucleus, a 

 curious anomaly must be mentioned. Some females of a strain of this kind 

 produce only female offspring, and yet the writer found that the eggs of these 

 females may have either 27 or 28 chromosomes in the equatorial plate of the 

 second polar division, so that, unless a chromosome is omitted in this division, 

 for which there is no evidence, the egg-nucleus must contain sometimes 27 and 

 sometimes 28, although eggs of the same parents, reared to maturity, produced 

 only females. 



2 SEILER has recently (1917) described a third case in the Psychid moth 

 Talaeporia in which the female has 59 chromosomes, the male 60, and the 

 mature eggs either 29 or 30. 



