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THE AMERICAN NATURALIST 



[^'oL. XLI 



embryos of the same sex, there must have been, at some period in 

 the development of the spermatozoon, a quahtative reduction of 

 the chromosomes, those necessary for a male going into one 

 spermatozoon, and those necessary for a female into another. 

 If we accept this hypothesis, we must show why a constant and 

 unequal proportion of all ova or of all spermatozoa have chromo- 

 somes, e. g. the accessory chromosome, which will produce an 

 embryo of a certain sex. As an alternative hypothesis we may 

 suppose that both ovum and spermatozoon play a sexual r6le, 

 and that the sex of the embryo, in common with other character- 

 istics, is determined by both sexual elements. This view, as I 

 shall show subsequently, is the more probable. 



If we cannot explain the cause of sex by postulating a series 

 of accidents of unknown nature occurring after fertilization, can 

 we explain it on the second possibility, — heredity? And if so, 

 which of the two great laws of heredity are applicable to the case ? 

 The first question I shall answer in the affirmative, and proceed 

 to the discussion of the second. 



The application of Mendel's law.— On the basis of Mendel's 

 law we must suppose that each ovum has equal chances of develop- 

 ing into a male or into a female embryo. Given two thousand 

 ova, chosen at random, the chances are even that a thousand of 

 them will develop into males and one thousand into females. We 

 might reasonably expect also that in some groups we would find 

 an excess of males, and in others an excess of females, but the 

 mean of all groups would be 1000 each of males and females. 



Punnett (:04b) in order to test Bateson's suggestion, attempted 

 an enumeration of the sexes in Carcinus moenas. He found an 

 excess of females in groups of individuals of the same size, but 

 this excess decreased in groups of younger individuals and there 

 were indications of an approximately equal distribution of the 

 sexes at the time of hatching. The exact proportion of the sexes 

 at the time of hatching could not, however, be determined. 



Mcintosh ('04,) from a study of the Norway l()]).ster. concluded 

 that the young were hatched in about equal |)r(»j)()rti(ni> nf tlie 

 sexes, but was not able to determine the exact pioportioii. 



Taking the English statistics given in Table III as a basis, we 

 may compute the probability that the actual distribution of males 



