SEX 369 



sexually or produce eggs which develop parthenogenetically into 



males. 



Another case in point is that of Dinophilus, in which abnormal temper- 

 ture conditions prevenl the laying and development of eggs from which 



the individuals of one sex normally arise. As Thomson ( L913, p. 502 

 remarks, ". . .if nutritive and other environmental influences .in- 

 operative, it is, in the main, by affecting the production and survival of 

 sexually-predestined germ cells." 



A clear case of sex-determination in the strict sense would be one in 

 which a given parthenogenetic egg, fertilized egg, spore, or young individ- 

 ual, in a unisexual organism, could be made to develop at will into either 

 sex;or in which such an organism, already showing the essenl ial characters 

 of one. sex, could be made to develop into the other sex (sex-reversal). 

 Evidence that in certain cases such a determination or reversal can 

 actually be accomplished has had much to do with the development of 

 recent metabolic theories of sex. 



Metabolic Theories of Sex — Animals. — According to the metabolic 

 theories of sex, the difference between the two sexes is primarily one oi 

 metabolism, the difference being not necessarily in the kind of metabolic 

 processes, as Geddes and Thomson thought, but more probably in their 

 rate or level. One of the most prominent exponents of this type of 

 theory is Riddle (1912, etc.), who has continued the important researches 

 on pigeons begun by Whitman many years ago. At each laying these 

 birds produce a pair, or clutch, of eggs. Under normal condition- the 

 first egg laid develops into a male and the second into a female. By a 

 careful analysis of a large number of eggs Riddle has been able to show 

 that the yolk of the first egg is the smaller, and has the smaller amount of 

 storage material (fats and phosphorus-bearing compounds), the higher 

 water content, and the higher oxidizing capacity, or higher metabolism. 

 The second or female-producing egg therefore differs in having a Larger 

 yolk, more storage material, less water, and a lower metabolism. The 

 chromosomes of the pigeon are not accurately known, but it is probable, 

 as indicated by the behavior of the sex-linked factors (see next chapter), 

 that the WZ type of sex-chromosome differentiation found in other bird- 

 is present here. Whatever their cytological differences may be. Ih< two 

 sexes are characterized in the egg stage by different levels of metabolism^ tin 

 male having the higher level and thefemaU the lower; and this difference has 

 been shown (as indicated by the metabolism of the blood) to persist into 

 the adult stage. 



The seasonal change in the amount of storage, and consequently in the 

 level of metabolism, is definitely con-elated with the percentage of a 

 given sex and the degree of it> manifestation. As the season advances 

 both eggs of the clutch -tore more energy-cunt aining materials, and the 

 birds developing from the second (female-producing) egg, while often 



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