MATERIAL AND METHODS Rats

As the first step in an investigation of the hormonal requirements of the pregnant rat, we [Rowlands & Williams, 1943 examined the conditions under which ovula-tion can be produced after hypophysectomy. An injection of 40 i.u. of serum gonadotrophin (pregnant mares ' serum) given 7-9 days after hypophysectomy brought the ovaries of immature or adult rats to a state where an injection of almost any gonadotrophic extract given 4-5 days later would produce ovulation. The next step has been to find out whether the eggs thus shed were fertilizable and sufficiently normal for their further development to be studied. The results of the examination are recorded below