534 FRANK R. LILLIE 



belonged mainly to the original territory of the sperm suspension, 

 and the ring owes its origin to migration of spermatozoa towards 

 the drop. They do not, however, penetrate at first to the center 

 of the drop, but their movements are arrested, hence the formation 

 of the ring. The ' tail' of the ring is due to migration of spermato- 

 zoa to the trail of CO 2 sea-water left behind in withdrawing the 

 pipette, leaving a clear zone marking the range of the effective 

 stimulus. Control: No reaction is given to a drop of pure sea- 

 water similarly introduced. 



The migration of spermatozoa to the first formed ring continues 

 for a short time; the ring thus grows broader and tends to close 

 in the center (fig. 4-2 and 3). Shortly after the ring and tail 

 aggregations have formed with reference to the introduced drop 

 of CO2 sea-water, the usual aggregations of the sperm, 1 to 2 mm. 

 in diameter, form in the remainder of the suspension outside the 

 drop evenly spaced throughout, if the sperm suspension is per- 

 fectly fresh (fig. 4-4). But if it is a little stale the general sus- 

 pension remains homogeneous. 



The detail of form of the ring and tail aggregations vary accord- 

 ing to whether the introduced drop simply displaces a certain 

 amount of the suspension, or is more or less mixed in the introduc- 

 tion; and this depends obviously on the size of the opening of the 

 capillary pipette and the rate at which the drop is introduced. 

 But the general form of the reaction is always the same. 



The spermatozoa in the CO2 aggregations are never in the least 

 agglutinated and their behavior is in all essential respects the same 

 as in the aggregations formed in any fresh suspension. I there- 

 fore early formed the hypothesis that the aggregation phenomenon 

 is a chemotactic reaction to CO2 produced by the spermatozoa 

 themselves, and this hypothesis has been abundantly confirmed, 

 as the series of experiments to be described will show. 



The formation of the described configuration in a suspension of 

 active spermatozoa with reference to an introduced drop is due 

 to positive chemotaxis to the drop. If the clear margin be ob- 

 served during the formation of the ring, the spermatozoa may be 

 seen swimming across it to the ring head first. Under the low 

 power of the microscope they appear to drift across it with a 



