iv DETERMINATION OF SEX 121 



why all fertilized eggs develop into females, is also quite unsolved. 

 Chambers (1913) described degeneration of large numbers of spermatozoa 

 in Simocephalus, and suggested that these, though not visibly different 

 from the others, were the male-producing spermatozoa. Taylor, however 

 (1915 b), found no evidence of degeneration of a whole class of spermatozoa 

 in the allied genus Daphnia. 



(8) The Relation between the Sex Chromosomes and the Determination 



of Sex 



This question, having been fully discussed in a recent publication 

 (Doncaster, The Determination of Sex, 1914), will be treated very 

 summarily here to avoid unnecessary repetition. 



In the cases where a dimorphism of the spermatozoa exists, it appears 

 plain that the sex of the zygote depends upon whether the egg was 

 fertilized by a spermatozoon with the X chromosome or without it ; 

 and similarly that where the spermatozoa are all alike and the eggs 

 dimorphic, it depends upon the nature of the egg which was fertilized. 

 It therefore seems legitimate to say that in these cases sex is determined, 

 or caused, by the sex chromosomes, but we must remember that the 

 sex chromosomes are only one of a number of causes. We have already 

 seen that in Phylloxera fallax a female gives rise parthenogenetically 

 through her descendants both to male-producing and to female-pro- 

 ducing eggs. It appears, therefore, that we must here look for an earlier 

 cause of sex than the presence or absence of the X chromosome 

 namely, something which determines whether the X chromosome shall 

 or shall not be eliminated at maturation. Similar considerations apply 

 to the hermaphrodite Ascaris nigrovenosus. 



In certain other cases it appears that the sex of the individual 

 can be determined by factors acting after fertilization, and at a time 

 therefore when we must suppose the chromosome equipment of the 

 embryo to be fixed. The experiments of King (1912) on toads, and 

 of R. Hertwig (1912) on frogs, make it probable that the sex of these 

 animals can be influenced by external factors acting on the egg either 

 before or after fertilization. An example from nature of the deter- 

 mination of sex by environment is afforded by the life history of the 

 marine worm Bonellia viridis (Baltzer, 1914). Here it appears to be 

 the environment of the larva which determines whether the adult shall 

 be male or female. 



Finally, it has long been known that the secondary sexual characters 

 of one sex may appear in individuals of the opposite sex as the result of 

 castration or other causes. 



Probably in every zygote, and indeed in every gamete, both sexes 



