THE FACTORS WHICH DETERMIiNE SEX 669 



cases even to the total exclusion of females. Kuschakewitsch ^ 

 showed that the results could not have been due to differential 

 mortality or differential fertilisation, and that seemingly, therefore, 

 the metabolic condition of the gametes must have been the cause of 

 the unusual proportions. King- also has shown that the sex-ratio 

 in toads can be affected by the metabolic condition of the eggs, as by 

 treating the eggs so as to reduce their water content more females 

 were produced, and by caiising the eggs to absorb water more males 

 were produced. 



Influence of Nutrition and Environment. — Schenk ^ also has 

 elaborated a highly speculative theory which supposes sex to be 

 determined by the relative degree of " ripeness " or " unripeness " of 

 the ovum ; but the term " unripeness " is here used to imply a 

 condition consequent upon incompleteness of nutrition, while 

 " ripeness " is held to result from a more favourable state of 

 nutrition. " Eipe " ova are supposed to develop into males, and 

 " unripe " ones into females. The presence of sugar in the urine is 

 evidence of an incomplete metabolism, and hence is regarded by 

 Schenk as implying a condition likely to result in the birth of 

 females. By supplying women with a highly nitrogenous diet, 

 which prevented the elimination of sugar in the urine and made the 

 metabolism more complete, Schenk claimed that he could ripen 

 the ova, and so increase the chances of male offspring. 



Cuenot* has described some experiments upon rats, in which 

 some were fed mainly upon bread, and others were given an abundant 

 supply of different Jinds of food. Schultze ^ did some . simiW 

 experiments upon mice, but in neither case was there any evidence 

 of a preponderance of the sex among the young of the better 

 nourished individuals. (For Seller's work on Talceporia, see p. 671.) 



It has long been known that parthenogenesis is the normal 

 method of reproduction among plant-lice or Aphides during the 

 months of summer, successive generations of individuals arising in 

 this way, but that with the approach of autumn males make their 

 appearance, and reproduction then becomes sexual. If, however, the 

 Aphides be kept in an environment of artificial warmth, and at 

 the same time are supplied with abundant food, the succession of 

 parthenogenetic females may be caused to continue for years without 

 the appearance of the sexual form. It is to be noted that the sexual 



1 Kusohakewitscli, Die EntwickluTigsgescMchte der Keimdriisen, etc., Festschr. 

 R. Hertwig, vol. ii., 1910. 



^ King, "Studies on Sex-Determination in Amphibians," IV., Biul. Bull., 

 vol. XX., 1911. The other papers in this series should be consulted. 



^ Schenk, The Determination of Sex, English Translation, London, 1898. 



* Cu6not, loc. cit. 



^ Schultze, " Zur Frage von den geschlechts-bildenden Ur.sachen," Arch. f. 

 Mikr. Anat., vol. Ixiii., 1903. 



