88 SEX INHERITANCE 
present in Abraxas that cytological basis which the 
evidence from sex linkage 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 fuliginosa. 
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). Itis 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; band 'c, daughter plates of the first polar spindle; d, equatorial 
plate of spermatogonium; e, equatorial plate of first spermatocyte; f 
and g, equatorial plates of second spermatocyte; h, anaphase stage of 
first maturation; 4 and 7, equatorial plates of somatic cells with 56 (in 2), 
and 61 chromosomes (in j). (After Seiler.) 
