40 



FERTILIZATION 



particular direction. After the photographs on which the tracks 

 were based had been taken, a glass pipette, filled with a i % solu- 

 tion of sodium L-malate in tap water (containing i% agar), was 

 inserted into the sperm suspension. Malate ions immediately 

 began to diffuse out of the pipette into the external medium; 

 capillary effects or hydrodynamic flow, which would have con- 

 fused the issue, were prevented by the agar gel. The response of 

 the spermatozoa to the malate gradient is shown in Fig. 8b, which 



. 16 /■ 2'4 



00 



0-1 



2 



T 

 0-3 



0-4 



0-5 0-6 



f (mm.) 



0-7 



0-8 



0-9 



10 



FIG. 8a. — Movements of bracken spermatozoa in tap water. The circles indicate 

 where each track begins. Numbers at the beginning and end of each track 

 refer respectively to time of start and duration of track in seconds. 



requires two comments. First, the spermatozoa move in a re- 

 markably purposeful way towards the source of malate. They do 

 not drift statistically towards the source, as might be expected if 

 the mechanism of attraction were of the type known as Klino- 

 kinesis with Adaptation, which occurs when Ullyott's 'Turning 

 Worm' is subjected to a light stimulus (1936). The conclusion is 

 almost inescapable that these spermatozoa have 'sense organs', or 

 their functional equivalents; but in this case the sense organs must 

 be efficient, because, in such a system, the differences in concen- 

 tration between the front and back ends of the head of a sperma- 



