Vol. XXX. January, 1916. No. i BIOLOGICAL BULLETIN CAUSES OF STERILITY IN THE MULE. 1 J. E. WODSEDALEK. (NINE PLATES.) CONTENTS. I. Introduction i II. Comparison of the Horse and the Ass : 3 III. Characteristics of the Mule 7 IV. Material and Methods 9 V. Appearance of the Testicular Tissue 9 VI. Spermatogonia 10 VII. Primary Spermatocytes 15 1. Early Prophase 15 2. Synapsis 17 3. Abnormalities in Mitosis 23 VIII. Giant Cells 32 IX. Multinucleate Cells 33 X. Destruction of the Germ Cells 34 XI. Summary 36 INTRODUCTION. For several years the author has been contemplating to make a study of the germ cells of the mule with the aim of offering a thorough explanation of the causes of sterility in this hybrid, but it was not until the past year that material for such an investigation was available. Cytological studies of the sex cells, especially those of animals, in recent years have added much to our knowledge of the mechanism of heredity; and such studies have kept fairly good pace with the comparatively rapid advances in our knowledge of plant and animal breeding which have been made since the rediscovery in 1900 of the results of Mendel. And while this study is now recognized as one of the chief methods of confronting many problems in genetics, and must be carried 1 From the zoological laboratories, University of Idaho.