380 



INTRODUCTION TO CYTOLOGY 



The XO type (Fig. 214) is exemplified by the moth Talceporia tubulosa. 

 In the female there are 59 chromosomes, including an unpaired X. In 

 some oocytes the X passes to the first polar body at /, while in others it 

 remains in the secondary oocyte nucleus. All of the chromosomes 

 appear to divide in //. Hence there are two classes of eggs, with 29 and 

 30 chromosomes respectively. In the male there are 60 chromosomes, 

 including an XX pair, and all of the spermatozoa are alike in having 

 one A''. Fertilization of the two kinds of eggs would therefore be expected 

 to give XO (9) and XX (cf ) (Seller, 1917). 



zo 



Fig. 214. — Sex-chromosome behavior in animals with heterogametic females. Division / 

 arbitrarily shown as disjunctional in all cases. ZO = XO; ZW = XY. 



Another moth, Phragmatobia fuliginosa, exhibits a modified XY 

 type of sex-chromosome behavior (Seller, 1913). In Abraxas grossulariata 

 the somatic nuclei of both sexes usually have 56 chromosomes, but the 

 females of some strains have only 55 and produce two classes of eggs 

 with 27 and 28, respectively (Doncaster, 1914). It is not unreasonable 

 to suppose that in the 56-chromosome strains the X is paired with an 

 inert Y and that the latter has been lost in the 55-chromosome strains 

 (Agar, 1920). 



