The Influence of Extraneous Factors 73 



epithelium in bulls has been stressed repeatedly by many investi- 

 gators (cf. Bratton et ai, 1948). 



'Dilution effect' and chemical changes associated with sperm senescence 



The changes which result from dilution of semen have been the 

 subject of much study; two distinct lines of research are recognizable. 

 The chief endeavour of the mammaUan semen investigators was to 

 solve the practical problem of the composition of artificial diluents, 

 whereas the workers interested in the sperm of lower forms, such 

 as the sea-urchin, were trying to establish the cause, rather than the 

 remedy, for the effect of dilution. It may be said at once that as yet, 

 there is no perfect semen diluent, and the precise mechanism of the 

 'dilution effect' still remains to be solved but much progress has 

 been made in both directions. 



Some of the early experiments on semen dilution, by Koelliker 

 (1856), Ankermann (1857), Engelmann (1868), and others, were 

 carried out with frog spermatozoa; in semen pressed out directly 

 from the frog testis, the spermatozoa were found to be motionless 

 but when mixed with a few parts of water, they became intensely 

 motile. This activity, however, was of short duration; it began to 

 decline already after a few minutes and seldom extended beyond one 

 hour. Prolonged motility was obtained when water was replaced 

 with 0-25 to 0-5% NaCl solutions. In the presence of higher concen- 

 trations of NaCl the spermatozoa remained motionless but could be 

 'revived', even after a relatively long time, by further addition of 

 water. It was also shown that the presence of oxygen is not absolutely 

 essential for the motihty of frog spermatozoa. This was first demon- 

 strated in 1868 by Engelmann at Utrecht who found that frog sperm 

 motility could be maintained for several hours in diluted semen, in 

 an atmosphere of hydrogen or carbon monoxide; however, when this 

 'anaerobic' motility began to decline, it was restored by pure oxygen 

 or air. 



The response of fish and sea-urchin spermatozoa to dilution with 

 water or salt solutions is not very different from that of frog sperm. 

 The addition of water or dilute salt solutions to trout semen pro- 

 vokes a shortlived burst of activity followed by gradual exhaustion 

 and death of the spermatozoa; oxygen has an activating effect on 



