MEASUREMENT OF SPERM MOTILITY 33 



in fairly straight paths. The heads of these sperm show periodic light 

 flashes in dark-field illumination but not in phase contrast. 



2. Cells which swim in small, almost closed, circular paths. These 

 sperm do not rotate and show no flashes off their heads. 



These two types of motile sperm will be considered in some detail. 



Rotating Cells 



The successive images of three cells are shown in Fig. 1. The flashes 

 off the heads are indicated by the fat, black heads. In these positions 

 the cell lies with the flat side of its head (in bull sperm, a flat elliptical 

 disk), perpendicular to the slide surfaces. The light beams of the 

 dark-field illumination are then reflected into the aperture of the 

 objective. In all other positions, light is scattered off the edges only 

 of the head, so that a ring is seen. This phenomenon is clearly due 

 to a rotation of the whole cell along its longitudinal axis. 



The wave which progresses along the tail of these cells is, re- 

 markably, synchronous with the rotation. This is seen in Fig. 1, 

 where, after a complete rotation, the whole cell returns to its original 

 position. The alignment of the tail with the head whenever it flashes, 



I 2 341 234 1234 1234 123 



HUKUH'KM 



89 10 I 2 3 4 5 6 7 8 9 10 I 2 3 4 5 6 7 8 9 10 



Fig. 1. Successive positions of three rotating cells (A, B, and C respec- 

 tively) at intervals of 1/50 sec. For each of the cells the positions are num- 

 bered; after each period of rotation the same sequence of positions recurs. 



