NOT ES AND LITERATURE 



IS THE FEMALE FROG HETEROZYGOUS IX REGARD 

 TO SE X-I ) E T E R M I X A T I O X .' 



Tiik evidence that sex is determined by an internal mechanism 

 in unisexual animals lias accumulated rapidly in the last few 

 years. The one outstanding case is that of the frog. That 

 extreme variations in the sex ratio occur in this amphibian has 

 been evident from the earlv experiments of Born 1881, Pfliiger 

 1882, and Yung 1881-85. The effects were generally ascribed 

 by the earlier workers to differences in the food of the tadpole 

 Most recent and more carefully controlled experiments, notably 

 those of Cuenot and of King, have shown beyond doubt that food 

 is not a factor that determines the sex of the tadpole. On the 

 other hand. Richard Ilertwig has effected astonishing changes in 

 the sex ratio of the frog by delaying fertilization of the eggs. 

 Over-ripe eggs produce a high percentage of males. This con- 

 clusion has been recently confirmed and extended by a student 

 of Hertwig's, Sergius Kuschakewitsch.' By delaying fertiliza- 

 tion of the eggs for 89 hours after the first eggs had been laid 

 (which gave 53 per cent, of males) there was produced 100 per 

 cent, of males. The death rate of the larvae was so low (from 4 

 to 6 per cent.) that it could not have seriously affected the results. 

 The following table gives the outcome of Hertwig's observations 

 and those of Kuschakewitsch. 



