In 1899 Gustav Born advanced the theory that the corpus luteum of the ovary is a gland of internal secretion, its activity being a prerequisite for the implantation and early development of the fertilized ovum. Conclusive proof of the theory was provided by Fraenkel ('10) in demonstrating that cauterization of the corpora lutea in pregnant rabbits leads to the resorption of the fertilized ova, provided that the operation took place during the first 7 days of gestation. He extended the period of the indispensable role of the corpora lutea in the …