SEX INHERITANCE 105 
kinds of males, one Xx, the other Xx’, and as 
stated, two kinds of female-producing sperm Xx 
and Xx’. 
Thus the life cycle is brought back to the starting 
point. It may be added that so far as the chromo- 
somes other than the X chromosomes are concerned 
there is no synapsis and no reduction to the haploid 
number in either line until the maturation divisions 
of the third or sexual generation occur. The life 
cycle of this species illustrates three points: 
First.—That all of the sperm are female-produc- 
ing, because the male-producing class of sperm de- 
generates, as has been shown by direct observation. 
Second.—That the parthenogenetic females can 
produce males through the elimination of two chro- 
mosomes. The female contains four sex chromo- 
somes and the male two. The elimination of the 
two chromosomes in the polar body of the male- 
producing egg has been directly demonstrated. 
Third—In this species the somewhat unusual 
relation of one stem-mother giving rise to the line 
that culminates in the sexual eggs, and of another 
stem-mother giving rise to the line that culminates 
in the males, can be explained on the assumption 
that one pair of the sex chromosomes is heterozygous 
in some factor indicated in the diagram by priming 
one of the x’s. This explanation is in part theoreti- 
cal, although it is based on the actual observation 
of two kinds of males that differ in respect to the 
behavior of one of the smaller x’s. 
In other species of phylloxerans, and in many 
