GERM-CELL HISTORY IN THE BROOK LAMPREY 93 



kewitsch ('10) that the percentage of males increased with the 

 length of time that fertilization was delayed. Kuschakewitsch 

 found that when fertilization was delayed as much as eighty- 

 nine hours all of the eggs developed into males. The mortality 

 among all the eggs in the culture was about 4 per cent. In the 

 case of the frog, an accessory chromosome has been described, 

 both by Levy ('15) and by Swingle ('17). Levy found twenty- 

 five chromosomes in the male germ cells of Rana esculenta. 

 During maturation division these were so distributed that half 

 of the cells received twelve and the other half thirteen chromo- 

 somes. The odd chromosome of the thirteen is the sex chromo- 

 some. Levy believes that the accessory chromosome undoubt- 

 edly has something to do with sex, but he thinks that it is not 

 the only sex-determining factor. He says: ''Man darf aber die 

 Geschlechtschromosomen nicht als den geschlechtsbestimmenden 

 Faktor bezeichnen, den sie sind nur die zuerst morphologisch 

 erkennbaren Zeugen einer stattgefundenen sexuellen Differen- 

 zierung." 



Swingle found the spermatogonial number of chromosomes to 

 be twenty-five in Rana pipiens. He found some cases in which 

 the sex chromosome divided during the second spermatocyte divi- 

 sion instead of during the first, and one case in w^hich the two 

 parts of the X-body were unequal in size. He thinks that there 

 may be some connection between the abnormality of chromatin 

 distribution which results, presumably, in the production of 

 three kinds of spermatozoa, and the fact that in certain strains 

 of the species, males, females, and individuals possessing marked 

 hermaphroditic tendencies occur. 



In the case of the frog it seems evident, both from the experi- 

 ments of Hertwig and Kuschakewitsch on delayed fertilization 

 and from those of Witschi ('14) on the effects of temperature on 

 the sex of the animal, that the accessory chromosomes, known to 

 be present, are not the sole sex determiners. Such a conclusion 

 is not a condemnation of the sex-chromosome theory. If other 

 factors also affect the sex of an individual, it shows that the sex 

 chromosome is but one of many such factors which may bring 

 about the same result. Temperature, for instance, may result 



