The writer of this volume worked for twenty years in association with the subject of the sketch. In the development of the biography Mrs. Mall and Dr. W. H. Welch helped greatly, as did also Drs. Simon Flexner and George L. Streeter. Medical men who studied their embryology with Mall at Johns Hopkins will find this book of special interest, but as a biography of a great teacher, written con amore, it will be attractive to all. In an appendix there are bibliographies and reproductions of correspondence referred to in the text. There are some minor errors in the names of persons and particularly the confusion of several middle initials which might easily have been avoided, for example, Frank S. Billings and James B. Angell.