220 RAYMOND PEARL AND H. M. PARSHLEY. 



than every individual fertilized early) and it is not possible to 

 make any accurate estimate of what the difference amounts to. 

 However, until some more definitely related factor is found it 

 may be tentatively concluded, purely as a working hypothesis, 

 that the observed changes in the sex ratios are correlated with 

 changes in the relative freshness (or staleness) of the ova at 

 the time of fertilization. 



The hypothesis that the metabolic condition of the ovum when 

 fertilized may influence the sex of the offspring finds support in 

 the recent work from Richard Hertwig's laboratory. Hertwig 1 

 in a series of studies on frogs found that over-ripeness of the eggs 

 was associated with a preponderance of males, extending in 

 favorable cases to the production of 100 per cent, males. These 

 results were criticized on several grounds. The experiments, 

 however, have been very carefully repeated and extended by a 

 student of Hertwig's, Kuschakewitsch, 2 who obtained similar 

 results. The experiments were criticized by Morgan first on the 

 ground of differential mortality, and second on the ground of 

 differential fertilization, but Kuschakewitsch was able to show 

 that the results were not open to criticism on these grounds. 

 Whatever the explanation the facts brought out by Hertwig 

 and Kuschakewitsch must certainly be accepted. The procedure 

 which they followed led to a great preponderance of males. 

 Miss King 3 has also shown that it is possible to modify signifi- 

 cantly the sex-ratio by changing the metabolic condition of the 

 eggs. 



In the last few years a considerable body of evidence has 

 accumulated showing that sex determination is primarily a 

 matter of inheritance. It is not necessary to review the volumi- 

 nous literature here. Essentially two points have been demon- 



1 Hertwig, R., "Ueber das Problem des sexuellen Differenzierung," Verhandl. 

 deutsch. Zool. Ges., 1905; "Weitere Untersuchungen iiber das Sexualitatsproblem," 

 ibid., 1906; "Weitere Untersuchungen," etc., ibid., 1907. 



2 Kuschakewitsch, S., "Die Entwicklungsgeschichte der Keimdriisen von 

 Rana esculenta. Ein Beitrag zurn Sexualitatsproblem," Festschrift f. Hertwig., 

 Bd. 2, 1910. 



3 King, Helen Dean, "Studies on Sex Determination in Amphibians, IV. The 

 Effects of External Factors, Acting Before or During the Time of Fertilization, 

 on the Sex Ratio of Bufo lentiginosus," BIOL. BULLETIN, Vol. XX., pp. 205-234, 

 1911. See also others paper in the same series. 



