CONTENTS: 
Introduction. 
General Arrangement of the Germinal Cells. 
Methods. 
A. Spermatogenesis of Normal Pigeons. 
Spermatogonia. 
Primary Spermatocytes: 
Early Plistory. 
Synapsis. 
Division. 
' Secondary Spermatocytes and their Divisions. 
Reduction. 
Transformation of the Spermatid into the Adult 
Spermatozoon. 
Summary. 
B. Spermatogenesis of Hybrid Pigeons. 
General Considerations. 
Abnormalities in Mitosis. 
Abnormalities in the Structure of the Sperma¬ 
tozoon. 
Degeneration of the Germinal Cells. 
The Individual Histories of the Sterile Forms. 
Conclusions from the Study of Hybrids. 
Suggestions. 
Summary. 
