PHYSIOLOGY OF THE SPERMATOZOON 103 



matozoa, which tend to accumulate in any region of 

 increased CO 2 tension. Any area of greater concen- 

 tration of spermatozoa, by producing more CO 2 than 

 other areas, becomes a center of aggregation, which, 

 when once begun, is bound to proceed to the limit on 

 account of increased CO 2 production. The reaction 

 cannot take place in the presence of excess of alkali, 

 owing to neutralization of the acid as fast as formed; 

 and a CO 2 tension of the entire medium sufficient to 

 inhibit movement will likewise prevent aggregation. 



It is obvious that the aggregation phenomenon can 

 be explained by assuming a positive orientation of the 

 spermatozoa in a CO 2 gradient. It has also been sug- 

 gested (Cohn, 1918) that it is due merely to gradations 

 of activity, and therefore presumably of the range of 

 unimpeded movement: activity decreases with rising 

 CO 2 tension; the spermatozoa in regions of higher CO 2 

 therefore tend to remain there, but those without, 

 owing to their wider range of movement, tend to move 

 into and remain within such regions. 



On the second assumption it is difficult to see how 

 it happens that the aggregations become so dense. 

 Neither are the spermatozoa motionless in such aggre- 

 gations as required by this theory, but they can be 

 actually observed to be exceedingly active around the 

 periphery of the aggregations, and their movements 

 are almost exclusively those of rotation such as are 

 exhibited in the thigmotactic response. 



In the attempt to analyze the matter farther, the 

 reaction of spermatozoa to solutions of CO 2 in sea-water 

 was tested by the writer by the Pfeffer capillary-tube 

 method, which consists of introducing capillary tubes 



