CLUSTER FORMATION OF SPERMATOZOA 129 



The same result is obtained if the motihty of the sperm is dimin- 

 ished or annihilated through the addition of KCl to sea water. 



These are all very striking demonstration experiments, which 

 leave no doubt that the cluster formation and the apparent sur- 

 face tension phenomena depend exclusively on the motility of the 

 spermatozoa. 



On the other hand, the writer convinced himself that in the 

 true phenomena of sperm agglutination, the motility of the sperm 

 is of no concern. We have mentioned the fact that the sperm of 

 Asterias, when it has been in 50 cc. sea water + 0.5 cc. ^ NaOH 

 for fifteen or twenty minutes, undergoes a real agglutination when 

 mixed with the supernatant sea water of different kind of eggs or 

 of cattle serum. This real agglutination takes place just as well 

 after the spermatozoa have been completely immobilized by KCN 

 as before. We may therefore be sure that the cluster formation 

 is not due to an agglutination. • 



CLUSTER FORMATION A POSSIBLE TROPISTIC REACTION 



The writer's idea of a tropism underlying the cluster formation 

 is at present only a mere working hypothesis about which it is 

 therefore not necessary to say much. It is briefly this, that the 

 spermatozoa which are rendered extremely active by the egg-sea 

 water are at the same time repelled by it, in other words, that they 

 possess a negative chemotropism or a negative differential sensi- 

 bility towards the egg-sea water; while any small or large mass of 

 spermatozoa at the boundary of or in egg-sea water acts as a center- 

 to which the isolated spermatozoa are positive. This would 

 account for the fact that the cluster formation is a function of 

 the motility of the spermatozoa and could also account for the 

 apparent surface tension phenomena. We should also under- 

 stand why the cluster formation (just like Lillie's 'agglutination') 

 lasts only a few minutes. Since the egg-sea water must gradually 

 diffuse into the mass of spermatozoa, the boundary at which they 

 are repelled must finally cease to exist. As soon as the concen- 

 tration of the active substance of the egg-sea water is the same or 

 almost the same in the cluster or the mass of spermatozoa as in the 

 surrounding sea water there is no more force active which may 

 induce or preserve the cluster formation. 



