22 



LORD ROTHSCHILD 



Direction of flow 



100 microns 



Fig. 2. Motion of the head of a dead bull spermatozoon in a parabolic 

 velocity gradient. The direction of flow was reversed 2.4 sec before the start 

 of the record. Individual sperm heads are one-eighth sec apart. Rate of 

 flow, 43 /xsec -1 , 15/jl from bottom of cell; velocity gradient, 2.6 sec -1 . 



hardly necessary to say that the position is more complicated and 

 different. To begin with, there is no doubt that both bull and human 

 spermatozoa swim upstream in a parabolic velocity gradient, if the 

 gradient is within the range 2-16 sec -1 . It is not at all easy to under- 

 stand how a spermatozoon can swim upstream in both halves of a 

 parabolic velocity gradient. If you throw me into a river at night, 

 there is no way by which I can tell whether I am swimming upstream 

 or downstream once my velocity is constant. One device for measur- 

 ing the stress on the front of my head would not help, though, as- 

 suming I am the size of a spermatozoon, two tangential stress gages 

 and a speedometer would do the trick. There is no known object, 

 including pathologically shaped idealizations, which will point in 

 the same direction in both halves of a parabolic velocity gradient. 

 Suggestions as to how a spermatozoon does it would, therefore, be 

 appreciated. A so-called simple explanation, recently put forward by 

 Bishop and Walton (1960), that the tail offers less "resistance" to the 

 flow than the head and is therefore carried downstream quicker is 

 fallacious. 



