The following is a report on observations made during the academic year 1960-61 while the author was engaged in study and research at the Institute of Reproductive Physiology and Pathology of the Veterinary College of Norway, Oslo. This activity was supported by a North Atlantic Treaty Organization Postdoctoral Fellowship under the direction of the National Science Foundation. A number of such fellowships are awarded each year in all areas of science to foster the understanding and exchange of information between scientists of the North Atlantic Treaty countries and to encourage the scientific careers of young men in the various fields of science.