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. 



