ACTION OF SEX HORMONES IN FOETAL LIFE 381 



The expectation may be brought more nearly into harmony with 

 the facts if it is assumed that in addition to ordinary fraternal (dizy- 

 gotic) twins, there are numbers of 'identical' (monozygotic) twins of 

 both sexes, and that while in the case of females these are both nor- 

 mal, in the case of a dividing male zygote, to form two individuals, 

 in one of them the sexual organs remain in the undifferentiated stage, 

 so that the animal superficially resembles a female and ordinarily is 

 recorded as such although it is barren. The records for monozygotic 

 twins accordingly go to increase the homosexual female and the hetero- 

 sexual classes, while the homosexual male class in which part of them 

 really belong, does not receive any increment. 



Cole thus tentatively adopts the theory, which has been worked 

 out most elaborately by D. Berry Hart, stated also by Bateson, 

 and implied in Spiegelberg's analysis ('61), that the sterile free- 

 martin is really a male co-zygotic with its mate. 



Cole's figures represent the only statistical evidence that we 

 have previously had on this subject. Let us follow his suggestion 

 and take from the two-sexed class enough cases to make the male 

 twins equal in number to the female pairs; this will be approxi- 

 mately one-fourth of the class, leaving the ratio 2:3:2 instead 

 of 1:4:2. Which one of these is the more satisfactory sex ratio 

 I leave others to determine; I msh only to point out the fatal 

 objection, that according to the hypothesis, the females remain- 

 ing in the two-sexed class are normal; in other words, on this 

 hypothesis the ratio of normal free-martins (females co-twin with 

 a bull) to sterile ones is 3:1; and the ratio would not be very 

 different on any basis of division of the two-sexed class that 

 wolild help out the sex ratio. Hitherto there have been no data 

 from which the ratio of normal to sterile free-martins could be 

 computed, and Cole furnishes none. I have records of 24 cases 

 statistically homogeneous, 3 of which are normal and 21 abnormal. 

 That is, the ratio of normal to sterile free-martins is 1 : 7 instead 

 of 3:1. 



This ratio is not more adverse to the normals than might be 

 anticipated, for breeders' associations will not register free- 

 martins until they are proved capable of breeding, and some 

 breeders hardly believe in the existence of fertile free-martins so 

 rare are they. * 



