Internal Factors of Sex Determination 423 



eggs and sperm contain both potentialities, and which is reaHzed 

 is determined by some internal relation that regulates the domi- 

 nance of one or the other sex. 



This idea leads to the following suggestion : — In Mendelian 

 hybrids of the first generation, in which two contrasted charac- 

 ters are present, it is assumed that when the germ-cells are pro- 

 duced half of them show already the dominance of one character 

 and the other half of the other character. This leads, after hap- 

 hazard matings, necessarily to the three types that appear in the 

 second generation. In the case of sex only two types are pro- 

 duced, hence it is not possible to imagine that two kinds of eggs 

 and two kinds of spermatozoa are produced.^ It seems, there- 

 fore, logical to conclude that the condition that leads to the de- 

 velopment of the alternative characters may exist in the egg 

 alone (as for the male bee), or in the sperm alone (as for certain 

 hemiptera), or by the combination of egg and sperm, as for the 

 female bee. In the last case the conditions that will lead to the 

 development of the male are found in the unfertiKzed eggs ; but 

 the addition of the sperm brings in a new condition that leads 

 to the development of the female — not that the spermatozoa of 

 the bee are female or even female-producing, but that the com- 

 bination of egg and sperm is female-producing. It is conceiv- 

 able that a non-nucleated egg of the bee fertihzed by a sperm 

 w^ould produce a male, the conditions being the same as in the 

 unfertilized egg. 



Admitting that all. eggs and all sperm carry the material basis 

 that can produce both the male and female, the two conditions 

 being mutually exclusive when development occurs, the imme- 

 diate problem of sex determination resolves itself into a study 

 of the conditions that in each species regulate the development 

 of one or the other sex. It seems not improbable that this regu- 

 lation is different in different species, and that, therefore, it is 

 futile to search for any principle of sex determination that is 

 universal for all species with separate sexes; for while the 



1 The assumption of reciprocal fertilization being rejected provisionally for 

 reasons already given. 



