The Germ Cells of Aphis. 327 



also in the male parthenogenetic egg, and there is no reduction of 

 chromosomes in either case. If the sex character resides in one 

 of the chromosomes, it is certainly not eliminated from the male 

 egg. How shall we harmonize the conditions in the Aphid with 

 Castle's theory.? We cannot argue as he does for Bombyx mori, 

 Ocneria dispar and all normally dioecious animals that there is no 

 uniform dominance of one sex over the other. How then shall we 

 account for the appearance of males, if all parthenogenetic Aphids 

 are sex-hybrids with the female character usually dominant.' 

 The only possible argument seems to be that certain favorable 

 conditions (warmth and abundant food) determine that the female 

 sex-character shall continue dominant, the male sex-character 

 recessive; while certain other conditions (not yet definitely deter- 

 mined) cause the male sex-character to become dominant and the 

 female character recessive. A similar change in dominance may 

 be imagined to account for the presence or absence of wings in 

 both the parthenogenetic and the sexual generations. According 

 to this theory we must suppose all of the oogonia to be alike in 

 their hereditary characters — all sex-hybrids as well as hybrids in 

 respect to other maternal and paternal characters. In addition, 

 we must suppose that in parthenogenetic eggs, which undergo no 

 reduction, dominance of certain characters can be reversed by 

 external circumstances. This may of course occur at a very early 

 stage in the history of the germ-cells, making the eggs at the time 

 of maturation virtually male or female. 



The case cited of two Aphids, each containing both partheno- 

 genetic and winter ovaries (Fig. 20), and also showing that a 

 parthenogenetic ovary may, after giving off parthenogenetic eggs, 

 change to a winter ovary, the sexual form, is strong evidence that 

 the ovarian oocytes of the Aphid may be affected by external con- 

 ditions. Whether the degeneration of certain oocytes in all 

 ovaries which are to produce winter eggs, can be considered evi- 

 dence that there are two kinds of eggs, those that produce partheno- 

 genetic young and those that require fertilization in order that they 

 may develop, is not clear. 



Beard ('02) says, in discussing parthenogenesis, "When in a 

 parthenogenetic form, a long series of one sex appears, the eggs 

 of the other sex must have been either delayed or suppressed." 

 According to Beard's theory we must suppose that in the Aphid 

 there are both male and female eggs, or at least such germ-cells. 



