77?^ Influence of Extraneous Factors 75 



of movement in undiluted semen to be due not so much to mutual 

 restraint, as to the presence of a specific sperm-immobilizing sub- 

 stance either in the sperm or in the seminal plasma. A suggestion has 

 been put forward that the main cause for the lack of motility is due 

 to the sperm-paralysing influence of androgamone I. Another line 

 of thought was that sperm quiescence is due to the seminal plasma 

 acting by virtue of its high potassium content or, alternately, its 

 low hydrogen ion concentration. 



The various theories concerned with the lack of sperm movement 

 in undiluted semen have been reviewed and analysed by Rothschild 

 (1948c, 1950^, \95\a,b). There is, he argues, no conclusive evidence 

 that the spermatozoa remain motionless in undiluted sea-urchin 

 semen because of any one factor such as allelostasis, sperm-immo- 

 bilization, pH, carbon dioxide or potassium ions. It is particularly 

 difficult, he points out, to reconcile the observations on the immo- 

 bihzing action of seminal plasma with the fact that seminal plasma 

 obtained by gentle centrifugation and used as a diluent for fresh 

 semen, renders the spermatozoa as active as sea-water. He found 

 that the seminal plasma of Echinus esculentiis acquires sperm- 

 immobilizing properties only after prolonged centrifugation of 

 semen, presumably as the result of leakage of an inhibitory sub- 

 stance from the cells; the rate at which the immobilizing substance 

 diffuses from the spermatozoa into the seminal plasma seems to vary 

 and this probably explains the conflicting reports concerning the 

 effect of centrifuged seminal plasma on sperm motility. Rothschild's 

 own experiments indicate that the main cause for the lack of sperm 

 movement in undiluted sea-urchin semen is deficiency of oxygen; 

 in fact, spermatozoa can be mobilized even in undiluted semen by 

 increased oxygen tension, and deprived of motility through oxygen 

 withdrawal. According to this author, the sudden outburst of 

 activity upon dilution of semen, is simply due to an improved 

 access of the spermatozoa to oxygen. 



As in frog and fish, the marked motility evoked in sea-urchin 

 sperm by dilution is only of limited duration and is subject to pro- 

 gressive decline in spite of the presence of oxygen. If the dilution 

 with sea-water is very excessive, the decline may almost immediately 

 follow the onset of activity. Gray (1928, 1931) thought that the 

 progressive loss of activity in dilute sperm suspensions could be 



