PREDETERMINATION OF SEX 313 



somes takes place, but that the members of each pair separate 

 before the chromosomes emerge. There is, however, no evidence 

 at present to show that conjugation does take place. 



It has been suggested by several writers that parthenogenesis 

 itself is due to the failure of the chromosomes to conjugate, but 

 such a view is in contradiction to the fact that in the egg of the 

 bee that produces a male and in the male-producing egg of Hy- 

 datina reduction oceurs and two polar bodies are formed, and 

 still the egg develops parthenogenetically. Furthermore, in ar- 

 tificial parthenogenesis it has been shown, first by Wilson, later 

 by others, that the embryo develops with half the full number of 

 chromosomes. Nevertheless, the fact remains that in cyclical 

 forms, where there is a parthenogenetic phase, the full number 

 of chromosomes is present, at least in forms that have adopted 

 this method as a means of propagation. There is no a priori 

 reason that we can give as to why a race might not develop 

 and perpetuate itself with the haploid number of chromosomes, 

 except that it would be made up only of males, because of the 

 presence of a single sex chromosome. For, in the only cases where 

 an animal develops with the haploid number of chromosomes 

 (namely, the bee and Rotifer) the individual is a male. 



THE LIFE HISTORIES OF CERTAIN APHIDS IN RELATION 

 TO PREDETERMINATION OF SEX 



In contrast to the life cycle of the phylloxerans of the hickories 

 the life cycles of most species of aphids is longer and in a sense 

 open, i.e., an indefinite series of parthenogenetic forms may oc- 

 cur, provided certain favorable external conditions persist. The 

 open phase of the life cycle lies between the stem-mother and 

 the sexuparae that bear the sexual males and females. 



The most important question in the aphids is whether there 

 are two lines starting with two kinds of mothers, as in Phylloxera 

 caryaecaulis, or only one line that splits later into the two sexual 

 types. Despite the extensive literature on the life cycle of the 

 aphids and their allies I can find only a few records that give the 

 necessary data to decide the question whether two or one Une 

 exists. If it were found that certain females produce sexual fe- 



