88 SEX INHERITANCE 



present in Abraxas that cytological basis which the 

 evidence from sex hnkage demands, namely, a con- 

 dition the converse of that known in other groups of 

 insects. 



The evidence that Seiler has obtained relates to the 

 wild strains of the moth Phragmatobia fuhginosa. 

 The reduced number of chromosomes in the polar 

 plate of the egg is 28 (Fig. 32, a). The large dyad 

 formed by synapsis of the sex chromosomes Z and W 

 is shown in the middle of the group. At the first 

 polar division all the chromosomes separate from 

 their mates, the ordinary chromosomes (autosomes) 

 as well as the sex chromosomes. But as W separates 

 from Z, it breaks into two parts which we may call 

 large W and small w (Fig. 32, b, c) . As a result there 

 are 29 chromosomes at one pole (the pole that con- 

 tains W and w) and 28 chromosomes at the other 

 pole (the pole containing Z). It is a matter of chance 

 which group goes into the polar body and which 

 remains in the egg. Consequently there are two 

 kinds of eggs, Ww and Z. 



In the male there are 56 chromosomes, which give 

 the reduced number 28. The two large Z's can be 

 made out in Fig. 32 , d. These meet, when the reduced 

 number 28 is formed, and then separate, one going 

 to each pole (Fig. 32, h). Each spermatozoon con- 

 tains, therefore, one Z chromosome. 



Fig. 32.— Phragmatobia fuliginosa. a, equatorial plate of first polar 

 body of egg ; b and c, daughter plates of the first polar spindle ; d, equatorial 

 plate of spermatogonium; e, ecjuatorial plate of first spermatocyte; / 

 and g, ecjuatorial plates of second .spermatocyte;/), anaphase stage of 

 first maturation; i and j, equatorial plates of somatic cells with 56 (in i), 

 and 61 chromosomes (in j). (After Seiler.) 



