GERM-CELL HISTORY IN THE BROOK LAMPREY 109 



one or the other mode of development is the result of a difference 

 in the body metabohsm in the two kinds of individuals, and is not 

 due to the inheritance of unalterable sex factors. If sex should 

 prove to be the result of shght differences in metabolism, it would 

 be easy to understand how a reversal of sex might take place 

 under certain circumstances. It must be admitted that these 

 metabolic differences might exist in the animals from the very 

 beginning of development, and, in so far as they are transmitted 

 by the parental germs which unite in fertilization, they may be 

 said to be inherited. If the sex characters are to be compared 

 with other so-called mendehan characters, we have to admit 

 also the possibility of a quantitative variation in the latter, which 

 seems to be contrary to the opinion of the majority of geneticists 

 at the present time. 



From evidence already presented we are forced to the conclu- 

 sion that in many dioecious forms, at least, every individual is a 

 potential hermaphrodite, in so far as it carries the latent quali- 

 ties of the opposite sex. Whether an animal develop into one or 

 the other sex may depend upon an inheritable quantitative rela- 

 tionship existing between the male and female potentialities in the 

 fertilized egg. As has already been mentioned, there seems to be 

 some evidence that in some forms it is one and in other forms it is 

 the other of the two sex cells that unite in fertilization which is 

 responsible for the quantitative difference of the sex factors. 

 The theory advanced by Castle ('03), that both male and female 

 cells are heterozygous with regard to sex, required the assump- 

 tion that selective fertilization was necessary in order to bring 

 about the observed results. This has been objected to on account 

 of the lack of evidence that there is such a selection among the 

 germ cells. Papanicolaou ('15) and Stockard and Papanicolaou 

 ('16) have brought forward some evidence that selective fertili- 

 zation might take place in the case of guinea-pigs, but the full 

 data have not, as far as I know, been published. 



If it be found necessary to consider one sex heterozygous for 

 sex and the other homozygous, the formula that appears most ap- 

 plicable is that adopted by Goldschmidt, which has been ac- 

 cepted with some modifications by Witschi and others. 



