Multiple pregnancies were obtained in cows treated with gonadotrophic hormones and the fetuses were removed 39 to 62 days after mating. This covers the period of sexual organogenesis in males from the initial differentiation of the testes to the final establishment of most male structures. Twenty-nine pregnancies resulting in heterosexual fetuses with fused chorions and vascular anastomoses provided forty-eight presumptive freemartins. The genital tract was studied histologically and gonadal size was determined by the paper weight method.
No difference was observed between control females and presumptive freemartins up to 48 days. Thereafter, growth of the presumptive ovaries practically stopped in freemartins and the diameter of the upper part of their Müllerian ducts decreased, as in males. Contrary to older views, the first freemartin effects occurred at the same stage (49 to 52 days) in all the presumptive freemartins.
Ovarian and Müllerian inhibition begins simultaneously and a significant correlation between the degree of inhibition of both structures was obtained. The penis, scrotum and prostate were absent in all freemartins until Day 60. In only one case was the ano-genital distance slightly increased. A delayed appearance of seminal vesicles was found in three out of seven freemartins at 61 and 62 days.
There was a notable absence of seminiferous tubules in the gonads of all these freemartins. It is suggested that if these tubules are present after birth it is because they have developed later.
The factors responsible for the freemartin effect still remain to be discovered. Ovarian and Müllerian inhibitions occur during the period of time when the external genitalia of males become masculinized. This suggests that a factor of male origin which inhibits both the ovaries and the Müllerian ducts and which is distinct from the masculinizing hormone could be involved. Other possibilities are also discussed.