32 ROBERT RIKMENSPOEL 



15% egg yolk citrate buffer (Rikmenspoel, 1957a). During transporta- 

 tion the semen was cooled to 4°C. Then a second dilution of 6 to 20x 

 was performed to make the concentration of the final suspension 

 suitable for measurements. 



If not mentioned otherwise, the standard conditions during meas- 

 urements were: pH = 6.7 ± 0.1, T = 37.1 °C, freezing point de- 

 pression A = -0.54 ± 0.02°C. 



OBSERVATIONS ON MOVEMENTS OF INDIVIDUAL SPERM CELLS 



Apparatus 



Most of the data reported here were obtained from moving films 

 taken at a speed of 50 frames /sec. Because of the special characteristics 

 of this apparatus, certain features should be specified (Rikmenspoel 

 and Van Herpen, 1956). Dark-field illumination was obtained by 

 means of a condensor with central stop. The light source was a 

 specially developed electronic flash bulb giving an exposure time 

 for each frame of about 150 /xsec (Robinson, 1954). This short ex- 

 posure per frame was sufficient to avoid blurring of the images of 

 the sperm clue to movement. A Reichert achromatic objective, XlO, 

 NA = 0.25, and a Reichert projection eyepiece were used, the latter 

 adjusted so as to give a magnification from sample to emulsion of 

 X20. A measuring chamber for the sperm sample 42 ± 2 microns 

 deep was used; this consisted of two slides each 1 mm thick, separated 

 by a plastic foil perforated by a hole 15 mm in diameter. About 40 

 microns should be considered the minimum depth of such a cham- 

 ber, if interference with the "vertical" tail movement is to be 

 avoided. The length of each film was about 30 sec. This is a suf- 

 ficient period to measure statistical data such as concentration and 

 velocity distribution of one sample (see below). 



Description of Movements of Sperm Cells 



Films obtained were projected frame by frame, which permitted 

 the consecutive positions of individual sperm to be drawn on paper. 

 By neglecting motionless cells and those which are clearly pathologic 

 (due to broken tails or strongly curved middle pieces), the cells 

 were found to fall into two distinct classes: 



1 . Cells which rotate along their longitudinal axes, while moving 



