We are especially grateful to Dr. Philip Corfman and his colleagues of the Population and Reproduction Grants Branch of NICHD for making this Conference possible. The format of this volume follows in general the order in which the papers were presented during the Conference. The Conference was divided into four sessions, each of which was presided over by a capable and distinguished scientist. Each of these chairmen, Drs. T. H. Hamilton, G. A. Puca, R. L. Vande Wiele and H. G. Williams-Ashman provided valuable discussion and for their services we are most appreciative. The Editors are indebted to Mr. Robert Colligan for his help in organizing