STUDIES OF FERTILIZATION 551 



has formed at the margin, and a clear zone arises external to it. 

 The ring is at first continuous, but it ruptures in numerous places 

 in two or three seconds and each segment contracts quickly to an 

 agglutination mass. 



The agglutinated masses in the interior of the drop are smaller 

 than in the ring, owing to the relatively low concentration of the 

 sperm suspension within the drop, and they break up very quickly 

 while the sperm is still extremely active. The movements of 

 the spermatozoa then gradually slacken; in a few minutes the 

 larger and more firmly agglutinated masses of the ring also begin 

 to break up and in ten or fifteen minutes all are resolved. 



This preliminary observation demonstrates a three-fold action 

 of the egg-extractive: (1) it activates the spermatozoa; (2) it 

 aggregates them through positive chemotaxis; (3) it agglutinates 

 them. The phase of increase of activity lasts only a short time, 

 a minute or two at the most, after which movements of the sperm 

 slacken and become less than the control, or cease entirely. Posi- 

 tive chemotaxis (aggregation) is shown by formation of a clear 

 zone external to the marginal ring. This is always a sign of 

 chemotaxis, as we have seen in the preceding section. 



The agglutination phenomenon is fundamentally different 

 from the aggregation; in the latter the spermatozoa are merely 

 loosely associated, and sHght agitation is sufficient to scatter them. 

 In the agglutinated masses the spermatozoa are stuck together 

 and are not separated by shaking. In the case of Nereis where 

 the agglutination is firmer than in Arbacia the masses may be 

 broken up into smaller masses by needles, or preserved en masse 

 in killing fluids. The breaking up of the ring into separate masses 

 is a characteristic agglutination effect ; the rings formed in response 

 to an acid do not break up unless the acid is very weak (see p. 537). 

 Finally an agglutinative substance produces its effect when 

 shaken up and evenly distributed in a vial of sperm suspension, 

 but an aggregative substance cannot of course exert a chemotactic 

 effect in the absence of a gradient. 



The same experiment succeeds well with Nereis. The eggs of 

 this form give off a substance (or substances) into the sea-water, 

 which causes aggregation and agglutination of the spermatozoa 



