PREDETERMINATION OF SEX ' 291 



small egg also gives off only one polar body (plate 1, g, h, i, j). 

 At this time the four sex chromosomes conjugate, so that two 

 whole chromosomes pass out into the polar body, as lagging 

 chromosomes, and two remain in the egg while all the other 

 chromosomes divide. The male that develops from this egg 

 has ten chromosomes. 



4. The sexual female produces but one egg, the reduction in 

 the number of chromosomes taking place at this time (plate 1, 

 /) . Presumably two polar bodies are given off, leaving six chro- 

 mosomes in the egg. 



5. In this species the male has ten chromosomes, two of which 

 are X chromosomes. During the first spermatocyte division 

 these two chromosomes pass into the functional cell, so that all 

 the functional sperm come to have six chromosomes. 



6. When these sperm fertilize the sexual egg the total number 

 of chromosomes is again brought back to twelve. 



THE CHROMOSOME CYCLE OF PHYLLOXERA CARYAECAULIS 



The main relations of the chromosomes in this cycle are illus- 

 trated in diagram 1. There are four ordinary chromosomes or 

 autosomes (colored black) and four sex chromosomes (repre- 

 sented by open circles). Two of the sex chromosomes are as 

 large as the autosomes and two .are much smaller. The latter 

 are in most stages loosely united to the larger sex chromosomes, 

 one to each. One of the small chromosomes is marked by two 

 cross lines in order to distinguish it from the other one. I have 

 marked it in this way because, as will be shown, an important 

 fact in the life cycle of this species can be accounted for, if one 

 of the two small sex chromosomes is different from its mate. 

 To the left in the diagram the line culminates in the sexual egg. 

 As seen to the right, the line derived from another stem-mother 

 (the one that contains the marked X) leads to two kinds of 

 males, each of which produces its particular class of spermatozoa, 

 one kind containing the open x and the other kind the marked x. 



If we first follow down the female line to the left, we see that 

 the egg laid by the stem-mother contains eight chromosomes. 



THE JOURXAL OI EXPERIMENTAL ZOOLOGY, VOL. 19, NO. 3 



