544 FRANK R. LILLIE 



This thigmotactic reaction may be the starting point apparently 

 of some of the aggregations formed in suspensions. Thus aggrega- 

 tions tend to form in the angle between the glass rod and the slide, 

 which appear to owe their origin to the thigmotactic reaction; 

 but when a considerable number of spermatozoa have accumulated 

 in the angle their CO2 excretion acts as a positive chemotactic 

 stimulus on the sperm of the suspension, and a dense swarm soon 

 forms along the rod, filling the angle and extending beyond it. 

 Such a continuous swarm then tends to break into evenly spaced 

 masses still in contact with the rod, owing to variations in CO2 

 production. Thus thigmotaxis in this case is the initial cause of 

 aggregations, which owe their subsequent growth to chemotaxis. 



It may be that the thigmotactic reaction is a frequent cause of 

 aggregations, particularly in suspensions that have produced 

 considerable COo when aggregations form only slowly and always 

 in contact with the substratum. But in fresh suspensions this 

 cannot be the case, for the aggregations first formed are freely 

 suspended. In many cases aggregations may be seen to form with 

 reference to firm strands or fibers of rnucus in a suspension, and 

 in such cases it appears probable that thigmotaxis and chemo- 

 taxis are combined. 



8. Variations of reactions 



So far as observed the behavior of sperm suspensions in sea- 

 water may be quite fully explained by the forms of reaction 

 described, and this brings the present section to a natural conclu- 

 sion. But we may finally note certain variations of the reactions. 

 The sperm suspensions were usually made, as stated, by mixing 

 a drop or two of dry sperm with about 8 to 10 cc. of sea- water in a 

 Syracuse watch crystal. This was done with a pipette, drawing in 

 the suspension and squirting it out again until the sperm was 

 evenly mixed. If this is done from one side, as is usually the case, 

 a current is made across the dish to the opposite side and back 

 along both sides, creating miniature whirlpools. Such currents 

 of course come to rest in a few seconds, but when the aggregations 

 become visible, ten to forty or more seconds later, they define 

 very accurately the original currents. At first I thought natur- 



