112 



CYTOLOGY CHAP. 



fact that she possesses an unpaired sex chromosome in her somatic cells 

 leads to the assumption that she produces two classes of eggs, of formulae 

 8 and 8 +X. All the surviving spermatozoa being probably of the 8 +X 

 class, it follows that the female is heterozygous and the male homozygous 

 for sex, as the phenomena of sex-Hnked inheritance in birds demand. 

 The cytological evidence, however, is in need of confirmation from other 

 species. 



In the Lepidoptera the cytological evidence of female heterozygosity 

 is stronger. The simplest case so far known is that of Talaeoporia tubulosa 

 (Seller, 1917). In metaphase I. of the female meiosis there are thirty 

 chromosomes. In anaphase, one of these lags behind the rest, but 

 ultimately gets included in one or other of the telophase groups, i.e. 

 in the nucleus either of oocyte II. or of the first polar body. This chromo- 

 some has evidently not divided, since the group which receives it has 

 30 chromosomes, while the other has 29. In metaphase 11. all the 

 chromosomes appear to divide. Thus two classes of eggs are produced, 

 one with 29 and the other with 30, or 29 + X, chromosomes. In the 

 male, 30 (bivalent) chromosomes appear in metaphase I., and all behave 

 ahke, so that all the spermatozoa have 30 chromosomes. Two types 

 of embryos were also found, one with 59 chromosomes, presumably 

 females (58 +X), and the other with 60, or 58 + XX chromosomes, which 

 are presumably males. 



It is interesting that in the anaphase of the first polar division the 

 unpaired X chromosome seems to go rather more frequently into the 

 polar body than into the oocyte nucleus, and that this corresponds with 

 the fact that females are more numerous than males in this species. 



In Phragmatobia fuliginosa (Seller, 1913) the heterozygosity of the 

 female is expressed in an unusual manner. The metaphase I. figures of 

 the two sexes are ahke, containing twenty-eight bivalents, one of which 

 is very much larger than the others ; this large one divides normally 

 in the male, but in the female the two chromosome groups formed in 

 anaphase I. differ from one another. One of them contains the expected 

 twenty-eight chromosomes, as in the secondary spermatocytes, but the 

 sister group contains twenty -nine chromosomes ; moreover, in this 

 group the large chromosome, though still much larger than any of its 

 fellows, is not so large as its mate in the group of the twenty-eight 

 chromosomes at the other end of the spindle, and Seller concludes that 

 the twenty-ninth chromosome has been produced by the breaking up 

 of the very large chromosome into a large and a normal-sized one. Thus 

 there is a physiological difference between the members constituting the 

 large bivalent in the female, for in anaphase I. one of them breaks up 

 into two and its homologue does not. This can clearly be compared 



