JSTEOTENY 1 and the sexual problem 



DR. W. W. SWINGLE 

 Department of Biology, Princeton University 



It has long been known that the larvse of certain Uro- 

 deles sometimes fail to undergo metamorphosis, yet be- 

 come sexually mature in the larval stage. Perhaps the 

 best known of such cases of neoteny, as this phenomenon 

 is called, is that of the Mexican axolotl, long regarded as 

 a separate species, now known to be an overgrown, sex- 

 ually mature larva of Amblystoma tigrinum. Several 

 other similar cases have been described, however. All 

 neotenous amphibians hitherto reported, with the excep- 

 tion of Allen's (1) thyroidless tadpoles, have been con- 

 lint'd to the tailed amphibians, and so far as the writer is 

 aware, the normal occurrence of precocious ripening of 

 the sex cells in larval Anura has never been described. 

 Oddly enough, it is the normal thing, and its occurrence 

 throws considerable light upon the obscure problem of 

 sex differentiation and development in the Anura, which 

 has long puzzled investigators of this subject, It will be 

 recalled that Pfliiger (5) reported years ago, that there 

 occurs normally in newly metamorphosed frogs three 

 kinds of individuals, males, females and hermaphrodites, 

 the two latter forms much more numerous in early stages 

 than the males. In the course of further development the 

 hermaphrodites become either definitely male or female, 

 as the sex ratio for adult frogs is approximately 50-50. 

 The investigations of R. Hertwig, Kuschakewitsch and 

 Witschi (2) not only confirmed Pfliiger 's work, but ex- 



