James Gray (1891–1975) was a British zoologist who was instrumental in establishing the field of cytology; he also worked extensively on animal locomotion. Originally published in 1931, this book was written by Gray as an introduction to his ideas regarding cytology and its relationship with zoology. The text constituted an attempt to reveal the complexity of the organism in terms of its relationship with 'physico-chemical laws', rather than restricting observation to its relationship with external factors. Illustrative figures are also included. This book will be of value to anyone with an interest in cytology and the history of science.