It is two years since a general meeting of the Gesellschaft fur Biologische Chemie first requested us to organize the 21 st Mosbach Colloquium on mammalian reproduction, and one year since we received final authorization to do so. The present volume contains the papers read at the Colloquium, but the discussions have been omitted because writing and proof reading them would have delayed the appearance of this volume for an unjustifiable long time. Besides, in most cases the discussion was of a relatively specific nature and we did not consider it essential, bearing in mind that the purpose of the Mosbach Col loquia is to provide advanced further education for the non specialist. One of us has referred to this and to the topical structure of the 21 st Colloquium in the introductory and final remarks. Helpful suggestions for organizing the program were made by some of the invited speakers, but the first important impulses VON BERSWORDT-WALLRABE, Dr. ELGER, Dr. came from Dr. GERHARDS, Dr. NEUMANN, and Dr. UFER to whom we here wish express our thanks. Thanks are also due to those whose donations, some of which were very generous, made it financially possible to organize the Colloquium. HEINZ GIBIAN July 1970 ERNST JURGEN PLOTZ Contents Introduction. H. GIBIAN (Berlin) 1 General Outline about Reproductive Physiology and its Developmental Background. A. JOST (Paris) .. 4 The Significance of Hormones in Mammalian Sex Differentia tion as Evidenced by Experiments with Synthetic Andro gens and Antiandrogens. W. ELGER, F. NEUMANN, H.