CYTOLOGICAL PROBLEMS 75 



Levy,* in Rana esculenta, and Swingle, in R. pipiens, describe 

 a sex-chromosome in the male, but their accounts are most 

 unconvincing. 



Loeb, however, puts forward the following possibility to 

 account for the dual sex-development. He assumes that the 

 female cannot be regarded as homozygous for sex, since in 

 that case the male would not possess the diploid number of 

 chromosomes. Though this seems a likely assumption, it 

 should be pointed out that since the exact chromosome number 

 was not determined in the material examined, homozygousity 

 of sex in the female cannot be definitely excluded. Assuming, 

 however, that the female is heterozygous, female frogs would 

 develop from eggs possessing the constitution 1 2 -j- X. Until, 

 however, exact cytological evidence is available regarding the 

 eggs from which females develop speculation is useless. 

 Further, the frog is a peculiarly difficult, and in some ways 

 unsuitable, animal for experimental work on sex-determination ; 

 for Hertwig,^ among others, has shown that changed conditions 

 can, to a large extent, affect the sex produced. Thus, in one 

 of his investigations, a frog was allowed to lay a few eggs 

 normally, after which she was removed and kept from the 

 male for sixty-four hours with the remaining eggs in the oviduct. 

 These were then fertilised by the same male. The first batch 

 of eggs gave a normal sex ratio, but the second batch gave a 

 great preponderance of males (700:100). A confirmatory 

 experiment by Kuschakewitsch, where the eggs were retained 

 in the oviduct for eighty-nine hours, gave a culture consisting 

 entirely of males. Control experiments showed that this was 

 not due to selective mortality of female ova or to over-ripeness 

 of the spermatozoa. Hertwig also showed that external 

 conditions could affect the sex ratio. Fertilised eggs kept at 

 30° C. gave 344 males and 319 females; but eggs kept at 

 i6°-i8°C. gave 260 males and 85 females. It was suggested 

 that over-ripeness of the eggs might cause the abnormal ex- 

 trusion of the female-determining factor — an X chromosome— 

 thus bringing about excessive male production. 



Morgan,* however, put forward the alternative suggestion 

 that the over-ripe eggs may develop parthenogenetically, either 

 by the egg-nucleus undergoing segmentation, or by the sperm 

 nucleus alone, giving rise to the segmentation nuclei. Such a 

 possibility is shown by the investigation of Oscar and Gunther 

 Hertwig,^ who showed that sperm nuclei, after treatment with 

 radium, can give rise to the nuclei of an embryo. 



1 Levy, F., Arch. Mikr. Anat., 2te Abt., 86, 1915, p. 85, 



2 Hertwig, R., Biol. Zentrabb., 32, 1912, p. i. 



^ Morgan, T. H., Physical Basis of Heredity (Lippencott, London). 

 * Hertwig, O. & G,, Arch. J. Mikr. Anat., 77, 1911. 



