riddle: control of sex ratio 



345 



and obtained (the latter author) in some cases a total of 100 per 

 cent males (Table 11). Dr. King ('12) did the converse of this 

 experiment with toad's eggs — withdrawing water from them be- 

 fore fertilization — and obtained nearly or quite 80 per cent of 

 females in cases where the mortality was less than 7 per cent. 

 The evidence afforded by these experiments on the frog and 

 the toad is thought by many to be inconclusive as evidence for 

 real sex control. Though selective fertilization has been elimi- 

 nated as a possibility by Kuschekewitch, we do not know which 

 is the heterogametic sex in amphibia and there also remains the 



TABLE 11 

 Experimentally Modified Sex Ratios in Frogs and Toads 



AUTHOR 





TREATMENT 



HESCXT 



tf 





C? 



9 





Hertwig, R 



60 



o 



- 

 — 



Delayed fertil. (+ H 2 0) 



271 







per eent 



100.00 

 88.88 



Kuschekewitch 



Delayed fertil. (+ H 2 0) 



299 







100.00 





-a 



03 



o 



No delay + H 2 



62 



41 



60.20 



King, H. D 



No delay - H 2 



106 

 85 



275 

 289 



27.66 

 22.73 



possibility of parthenogenetic development to account for the 

 excessive male-production in the experiments with the frog. 

 But this appeal makes it impossible to explain the great excess 

 of females obtained by Dr. King on the eggs of the toad, where 

 a selective mortality is definitely excluded, and leaves such 

 doubters to lean upon the rather discredited staff of selective 

 fertilization — a proposition wholly disproved for the related frog 

 and for the pigeon. It may be noted, however, that on the basis 

 of our present knowledge of the "sex-differentials" (to be con- 

 sidered later) in the pigeon's eggs both of these experiments 



