HEREDITY AND SEX 



gous. In such cases there is reason to think 

 that structurally the male is 2 N but the female 

 2N +. That is, the female is still the equiva- 

 lent of the male plus some additional element 

 and function. A structural basis in the chromo- 

 somes for such a condition has been described 

 by Baltzer in the case of the sea-urchin. He 

 found the regular duplex number of chromo- 

 somes in the male; but in the female, while 

 the number was the same, one of the chromo- 

 somes was larger than its mate, having an extra 

 or odd element attached to it. In such a case 

 the gametes formed by the male would all be 

 N, but those formed by the female would be 

 of two sorts equally numerous, viz. N and N + 

 (see Fig. 51). Egg N fertilized by sperm N 

 would produce a zygote 2 N, a male ; egg N + 

 fertilized by sperm N would produce a zygote 

 2 N +, a female. Hence here, as in other ani- 

 mals, the sexes would be approximately equal, 

 but the sex of a particular individual would 

 depend upon which sort of egg gave rise to it. 

 Upon the existence, as in the foregoing cases, 

 of an unpaired or odd structural element in 

 the egg, may perhaps depend the explanation 



169 



