RELATION TO GENERAL BIOLOGICAL PROBLEMS iii 



to two kinds of spermatozoa, male-producing and 

 female-producing. In still other cases, notably the 

 hymenoptera, males are produced when the eggs are not 

 fertilized and females when the eggs are fertilized. All 

 of these apparently divergent phenomena are consistent 

 with the idea that sex is determined in the germ-cell 

 and that the sex-determining factor is in some way 

 intimately associated with the presence of a peculiar 

 chromosome (the X chromosome), or group of chromo- 

 somes, in the nucleus of the germ-cell. This mechanism 

 gives a fex-bias to the individual, a bias in some cases 

 so strong that no known factors can interfere with the 

 fulfilment of the sex-development that was originally 

 determined. In other cases, however, sex may be 

 zygotically determined, but requires a definite favorable 

 environment to bring it to complete development or 

 differentiation. Finally, in some cases the individual 

 which is zygotically sex-determined may have its 

 sex-development so altered as to become largely of the 

 opposite sex. 



In mammals there is much evidence that sex is 

 zygotically determined; there appear to be two kinds 

 of spermatozoa and but one kind of egg, and the sex of 

 the individual depends on whether a male-producing or a 

 female-producing spermatozoon fertilizes a particular egg. 



If then sex in mammals is determined in the unde- 

 veloped egg, we should naturally expect twins or large 

 numbers of individuals derived from a single egg to be 

 of the same sex. This is just the point upon which 

 monozygotic twinning and polyembryony have a 

 bearing, for in these phenomena we have experiments 

 demonstrating the theory of zygotic sex-determination. 



