Acknowledgments

Dr. A. J. Dalton, now of the Sloan-Kettering Institute for Cancer Research, New York, gave indispensable help with the electron microscopy, and kindly handed over those micrographs which did not cover our previous joint work on Lumbricus. The present micrographs were made at the Cancer Department of the U.S.A. Institute of Health, Education and Welfare at Bethesda, Maryland. The earlier studies of this material were partly made at the Argonne National Laboratory, Lemont, U.S.A., under the auspices of the U.S. Atomic Energy Commission. Dr. Bernard Nebel, Dr. Roth, and Mrs. Rosemarie Devine of this Laboratory have given some assistance—Dr. Nebel kindly advised me on the maturation phase micrographs, Mrs. Devine has allowed me to refer to her unpublished work on Melanoplus differentialis, and Dr. Roth went over his micrographs of lepidopterous germ-cells with me. The osmium tetroxide used in this research was purchased with a grant from the Irish Medical Research Council. This paper was written in New Zealand during leave granted by the Board of Trinity College, Dublin, as Research Professor of Zoology

and Comparative Anatomy. Professor Richardson, of Victoria University, kindly gave me all necessary facilities in his laboratory.