Plate XV. The movement of spermatozoa of the sea urchin 

 and bull. 



{a, b and c) Photographs of sea urchin sperm showing the 

 form of the bending waves of the tail. (Exposure :Tno sec). 

 From Gray (1955). About ' 1000. 



{d) Photograph of a sea urchin sperm showing the " optical 

 envelope " traced out by the moving tail. (Exposure -ir, sec). 

 From Gray (1955). About x 1000. 



{e) A succession of photographs of a moving bull sperm at 

 sV sec intervals, showing the " optical envelope " traced out by 

 the tail. From Gray (1958). About : 800. 



if-m) Eight frames of a cine film of a moving bull sperm 

 taken at intervals of rV sec. From Gray (1958). About • 500. 



