STUDIES OF FERTILIZATION 535 



dancing motion like motes in a sunbeam owing to their spiral 

 path. If the external edge of the clear zone be carefully observed, 

 the spermatozoa can be seen to detach themselves one by one 

 from the general suspension and pass straight over to the ring. 

 But only those freely suspended make the direct path; those in 

 contact with the slide or cover continue their circus movements; 

 the chemotactic stimulus seems unable to overcome the thigmo- 

 tactic reaction. 



The reaction is given most clearly and rapidly by a fresh sperm 

 suspension, although one which has passed the aggregation stage 

 still gives it; however, as the sperm suspension becomes stale 

 the reaction becomes slower, and eventually ceases. Spermato- 

 zoa killed by gentle heat give no such reaction, thus excluding any 

 purely physical diffusion effect as cause of the phenomenon. 



In the case of this reaction in a somewhat stale non-aggregative 

 suspension the movements of the spermatozoa on the outer mar- 

 gin of the ring are decidedly more vigorous than in the general 

 suspension. This would appear to indicate that, at this place in 

 the CO 2 gradient marked by the clear zone, the concentration of the 

 CO2 is stimulating rather than depressing; but when we consider 

 that the CO 2 gradient must rise from the suspension across the 

 clear zone to the ring, and that the relative inactivity of the sperm 

 in the suspension is due, partially at least, to CO2 the conclusion 

 is not so clear. In any event, if we attribute a stimulating action 

 to a given CO2 concentration on such evidence, we must regard 

 the depression of activity in the general suspension as due partly 

 to other excreta. 



The conditions established by the experiment may be repre- 

 sented diagrammatically as follows (fig. 5). The injected drop 

 is represented by the continuous line circle and continuation, the 

 general suspension by the shaded area. By diffusion from the 

 injected drop a CO 2 gradient is established outwards, and this must 

 extend into the drop a certain distance because the gradient is 

 established by loss of CO 2 from the drop. The concentric broken 

 lines represent the gradient, or at least that part of the gradient 

 which is affective in the reaction. The thick open circle and the 

 similar linear extension represent the aggregations of the sperma- 



