CASTLE: THE HEREDITY OF SEX. 195 
the normal hermaphrodite, probably produces mosaic gametes, ¢ 9, for 
when in fertilization these unite in pairs, they invariably form hermaph- 
rodite individuals, @?. If segregation occurred in the production of 
the gametes, we should expect the occurrence also of its counterpart, 
dominance, in fertilization. Since in hermaphrodites the latter does not 
occur, it is probable that the former does not occur either. 
But in dioecious species sexual dominance almost invariably occurs ; 
it is probable, therefore, that in such species segregation of sex-char- 
acters takes place in the formation of the gametes. If so, and if, as in 
color heredity among mice, all possible combinations of gametes are 
formed in fertilization, and in the frequencies demanded by the law of 
chance, the sex of the offspring should be indicated by the product, — 
36+? (ova) 
&é + 2 (spermatozoa) 
6$4+2924+ 22. (zygotes). 
According to this, half the offspring, it will be observed, must be purely 
of one sex or the other ; that is, must contain and transmit the characters 
of one sex only. But we have no reason to think that such sexually 
“pure” individuals exist. On the contrary, when, as in the case of the 
honey-bee, the individual apparently transmits uniformly the character 
of one sex, that sex is invariably the opposite to its own. It is highly 
probable, therefore, that an egg bearing the character of one sex can 
unite in fertilization only with a spermatozodn bearing the character of 
the opposite sex. Our present knowledge of the process of fertilization 
indicates that in it a union is accomplished between elements strictly 
equivalent to those which were separated in the formation of the 
gametes. But there exist, as we have seen, strong reasons for believing 
that in the formation of the gametes, opposite sex-characters are sepa- 
rated. Consequently, on a priort grounds, we should expect only 
opposite sex-characters to unite in fertilization. 
But, some one may object, if a ripe egg of one sex can be fertilized 
only by a spermatozo6n of the opposite sex, it follows that half the eges 
produced are infertile toward half the spermatozoa. This, however, is 
not so serious an objection as it may at first thought seem to be. It 
does not involve impotency of half the eggs and spermatozoa, nor of any 
portion of them. All the eggs of one sex will be fertile toward all the 
spermatozoa of the opposite sex; the remaining eggs will be fertile 
toward the remaining spermatozoa. The infertility which exists is only 
