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 hne 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 ehmination 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 pliylloxerans, and in many 



