This work of Goldschmidt shows definitely that intersexuality in Lymantria depends upon variations in the relative rate of production of definite instances and that this again is correlated with differences in the sex-determining factors that can be inter preted in Mendelian terms. (See HEREDITY.) In the case of the sexually abnormal types of Drosophila it is seen that the abnormality is the result of a disharmony in the distribution of the elements of the hereditary constitution of the individual. In the case of Lymantria intersexuality and sex reversal are due to a disharmony in the composition of the hereditary constitution. The sexually abnormal individuals of Drosophila are spatial intersexes; those of Lymantria and of the frog and of the small Crustacean Gammarus (Sexton and Huxley, 1927), are consecutive intersexes, sex-mosaics in time.
No spatial intersex is possible in mammals since the control of sex-differentiation lies not in the cells but is relegated to the sex-hormone. No certain case of consecutive intersexuality is known in mammals although there is no theoretical objection to its occurrence. But intersexuality, though its causes may not yet be exactly recognised, does exist quite commonly.
One form of sexual abnormality possibly results from delayed hormone-production. In man and in most domesticated mammals a peculiar type of sex abnormality is sometimes encountered.
Examination shows (I) that testes only are present but often occupy the position of ovaries or are at least abdominal; (2) that both male and female sets of internal accessory sexual organs are present; (3) that the external organs of reproduction are mainly female in type; (4) that male secondary sex-characters and instincts develop at puberty. Crew (1923) has adduced reasons for regarding these abnormal forms as males in which, for some reason, the testis has not been secreting its controlling hormone during embryonic life or at least until the main internal and external accessory sexual organs have been laid down. This interpretation presupposes that the differentiation of the acces sory organs occurs during a limited period of embryonic life and that the process is more or less irreversible. In the absence of testicular hormone both male and female accessory organs can become differentiated up to a point and thus a neutral type results. The various accessory organs have slightly different periods of differentiation and according to this and to the degree of delay in the production of testicular hormone the individual will be intermediate or predominantly male. Such individuals owing to the appearance of the external organs of reproduction are usually classified as females at birth but prove to be males at puberty. The condition is hereditary.