This small volume consists of a series of papers presented in 1957 at Kings College Hospital Medical School in London. A brief historical introduction by Dr. Murray L. Barr describes the circumstances that led, some eight years earlier, to his recognition of sexual dimorphism in the nuclei of mammalian cells. The first seven papers are devoted to the morphology and the genetic significance of the so-called sex chromatin, and the remaining papers deal with applications of "nuclear sexing methods" in clinical medicine and include discussions of sex chromatin in congenital errors in sex development, in teratomas, and in tumors. Each paper is accompanied by a bibliography and a discussion, and there are numerous illustrations throughout the text, including many well-reproduced photomicrographs. This book can be recommended as a stimulating introduction to this fascinating and rapidly developing field.