688 THE PHYSIOLOGY OF REPRODUCTION 



above alternative conclusions improbable. Whether the second or 

 third of the other possible conclusions is to be accepted must remain 

 doubtful so long as we are not in a position to estimate the quantitative 

 effect of the factors given above. From the necessarily rough estimate 

 which he has been able to form, the writer's opinion is that their 

 combined effect would not be sufficiently great to mask a pre- 

 ponderance of female births due to better nutrition, and consequently 

 he is inclined to believe that in man, at any rate, the determination 

 of sex is independent of parental nutrition. In any case its influence 

 can be but small." 



Newcomb,^ as a result of an investigation into the statistics of 

 multiple births, has come to the conclusion that sex is established at 

 different periods of development in different cases. He shows that 

 there is a tendency among human offspring for twins to be of the 

 same sex, a fact which he regards as supplying a "practically 

 conclusive negation of the theory of completely determined sex in 

 the original germs." . His conclusion appears to be that sex is 

 established by " accidental causes," the nature of which is at present 

 unknown, and that in the case of twins the sex-determining factors 

 act similarly on both children, and so tend towards a uniformity of 

 sex. But he omits to mention the probability that some twin 

 embryos arise from a single ovum, a fact which would account for 

 their sexual identity on the assumption that sex is already determined 

 in the germ-cell. 



Hermaphroditism and Sexual Latency 



Organisms which combine, within themselves the essential 

 characters of both sexes are said to be hermaphrodite. True 

 hermaphrodites produce both ova and spermatozoa, but there are all 

 gradations between true and partial hermaphroditism (in which the 

 essential organs of reproduction are not involved), and between the 

 latter and the completely unisexual condition, in which the characters 

 of the other sex are either latent or absent altogether. 



Complete hermaphroditism is the normal state in many groups of 

 invertebrate animals (many sponges, coelenterates, and worms, and 

 some molluscs and crustaceans). In some forms the male and female 

 sexual elements do not exist contemporaneously, but are called forth 

 separately by different environmental conditions or are associated 

 with particular phases in the reproductive cycle (see Chapter I.). 

 In such cases the fact that the animal is hermaphrodite is liable to 

 be obscured. 



Among vertebrate animals true hermaphroditism is rare, though 



' Newcomb, loc. cit. 



