538 FKANK R. LILLIE 



a greater CO2 concentration within the ring, which furnishes 

 centers of aggregation positive to the 1/200 concentration of the 

 drop. Drops below this concentration or drops of pure sea-water 

 furnish no reaction. 



Summarizing; it would appear that the spermatozoa of Nereis 

 follow a CO2 gradient to the point of paralysis (about 1/100 sat- 

 uration). The clear zone outside an aggregation represents the 

 effective CO2 gradient in every case. The various forms of 

 reaction to drops of different concentrations follow from this 

 simple principle. 



2. Interpretation of the aggregaiion reaction 



We are now prepared for the interpretation of the aggregation 

 phenomena exhibited by fresh sperm suspensions described on 

 page 519. The spermatozoa as they come from the body cavity 

 are absolutely quiescent; as soon as they are suspended in sea- 

 water they become intensively active, and consequently produce 

 CO2 very rapidly. Any area of greater concentration of sperma- 

 tozoa, by producing more CO 2 than other areas, becomes a cen- 

 ter of attraction, and aggregations of the spermatozoa once begun 

 are bound to proceed to the limit, because the closer the aggre- 

 gation the greater the CO2 production and consequently the 

 greater the chemotactic stimulation. If aggregations once formed 

 are broken up and the spermatozoa evenly suspended once more, 

 the CO 2 tension in the suspension is greater than at first and is 

 evenly distributed. Hence, in the first place the activity of the 

 spermatozoa is reduced, and, in the second place, the differential 

 of the^ gradient between that of the general suspension and the 

 point of paralysis is greatly lessened. Therefore aggregation 

 takes place more slowly and less completely than before; and, 

 after a second and a third stirring up, the CO2 tension in the entire 

 suspension has become too great to permit of sufficient activity 

 to react to the slight possible differential gradient. 



It is obvious that such a reaction can take place only in the 

 case of spermatozoa that exhibit extreme sensitiveness to CO2. 

 The spermatozoa of Nereis possess by far the greatest sensitive- 

 ness to CO2 of any studied, as we have already seen. * No other 



