1 8 STUDIES ON THE GERM CELLS OF APHIDS. 



females, while those from other lines gave either only males or both 

 males and females, the natural conclusion would be that some fertil- 

 ized eggs contain the male character, while others do not. If, on the 

 other hand, both males and females should occur in all the lines of 

 descent, we should feel reasonably sure that all the fertilized eggs 

 contain both the male and female character. 



Unfortunately there is no such visible dimorphism in the sperma- 

 tids of the aphid as in some of the Hemiptera, Orthoptera, and Coleop- 

 tera to serve as direct evidence of sex -determining power, but if the 

 sex characters are represented in the chromatin as Mendelian alter- 

 nates there is abundant evidence that the spermatogonia must con- 

 tain both characters and that the spermatozoa must be pure as to the 

 two alternating characters. 



As to which seems the more likely supposition (i) that only germ 

 cells with opposite sex characters can unite to give a fertile egg, or 



(2) that all the winter eggs can be so maturated that only the female 

 character remains, one proposition seems about as likely to be true as 

 the other ; and at present I see no third possibility, since there is no 

 evidence that the female element is removed from the parthenogenetic 

 eggs that produce males (Castle, '03) and the spermatozoa must there- 

 fore be of two kinds respecting the sex characters, just as in the case 

 of any other two homologous maternal and paternal characters.* 



Whatever evidence isolation experiments may give on this point ot 

 the distribution of the male and female characters in the fertilized egg, 

 we know that from one parthenogenetic egg can come an individual 

 whose germ cells can produce (i) parthenogenetic, (2) sexual female, or 



(3) male offspring, and so far as we can see the indications are that 

 the same influences that cause a change in the mode of reproduction 

 from parthenogenetic to sexual produce in some species or in some 

 individuals of a species a definite change in dominance of the sex 

 characters (Oenothera aphid No. I and white rose aphid), while in 

 other species (goldenrod and willow aphids) a more or less balanced 

 state of dominance must be associated with the initiation of sexual 



* In the case of Aphis oenotherae it is possible that the germ cells of the red 

 parthenogenetic individuals contain the female sex character in both the paternal 

 and the maternal series of chromosomes, while the green individuals have the female 

 sex character in the maternal series and the male in the paternal series, the green 

 color being correlated with the male sex character and dominating over the maternal 

 red. The former line would then give rise to sexual females and winter eggs con- 

 taining only the female character; the latter, by a change in dominance of the sex 

 character, would produce males containing both characters. Fertilization by the 

 resulting two classes of spermatozoa would then perpetuate the two sexual lines. 



