Structure of the spermatozoon was studied by staining intra vitam under the cover-slip, and by methods of fixation and specific staining following Schumacher''s methods for lipoids and lipoproteins. The nucleus and axial tail filament, decidedly hydrophile and lacking in lipoids, are surrounded by a cytoplasmic layer rich in lipoids (lipo-chromatic substance) which assumes different relationships in the head at different times related to different functional stages and to differences in the medium. From the region of the middle piece this substance may send out delicate motile pseudopodial processes (sensillae amoeboideae). In egg-water the spermatozoon undergoes transformations resulting in extrusion of a round body from the head part that takes up a lateral position (lateral body).