DETERMINATION AND INHERITANCE OF SEX 547 



assume that all the eggs possess a predominating male tendency. There 

 appears to be Justification for this assumption since two polar bodies are 

 formed and a reduction division takes place which may be thought of as 

 partially eliminating the female tendency. Due to the preponderance 

 of female elements, fertilized eggs become females. Unfertilized eggs 

 become males, since the male tendency is undominated. 



Aphids produce brood after brood of parthenogenetic females while 

 food and climatic conditions remain favorable. When environmental 

 conditions become adverse both males and females appear between whom 

 eggs are fertilized, which as " winter eggs " resist the elements until the 

 following summer, when they develop into parthenogenetic females. 

 These facts may be thus explained: the original ancestor (stem-mother) 

 of a parthenogenetic series arises from a fertilized egg ; the eggs of the 

 parthenogenetic females are all hybrids as to sex. No reduction is 

 thought to take place when the single polar-body is formed. When 

 conditions are favorable, metabolic activities give the ascendency to the 

 female elements; when these become adverse, the male element gains 

 the ascendency in *some of the eggs. When males and females appear 

 the sex determinants are segregated in the ova and spermatozoa in equal 

 proportions in each. During maturation in the male aphid, however, 

 Dr. N. M. Stevens, of Bryn Mawr College, and also Dr. W. B. von 

 Baehr, of Germany, have discovered that half the sperm degenerate 

 (these lack the accessory chromosome) and that these are male-pro- 

 ducing kind. Hence, since only female-producing sperm remain, all 

 fertilized eggs (sex-hybrids) must develop into females. 



In the case of Phylloxera, where many generations of wingless 

 parthenogenetic females appear while environmental conditions re- 

 main favorable, and when these become adverse, give way to winged 

 males and females. Dr. T. H. Morgan has discovered a similar degen- 

 eration of the male-producing spermatozoa and the loss of a chromosome 

 in the parthenogenetic males. In these cases we must assume that the 

 eggs are intrinsically different (male and female), as they really appear 

 externally, or else, as Correns proposes, that the eggs are potentially 

 male and that in the fertilized egg where both sex tendencies are present 

 femaleness dominates when environmental conditions are favorable to 

 constructive metabolism. But some mechanism or condition must 

 remain whereby an apparently homozygous male may produce sperma- 

 tozoa bearing a female tendency. It is by no means proved that the 

 whole quota of female tendencies (elements) is eliminated at matura- 

 tion. The male and female (paternal and maternal) chromosomes are 

 probably promiscuously arranged on the spindle. The persistence of 

 chromosomes bearing female characters may supply the demands for the 

 production of female-producing spermatozoa. 



The sex of the offspring then appears to be the result of the inter- 



