532 FRANK R. LILLIE 



as are exhibited by suspensions of Nereis spermatozoa. But indi- 

 cations of the same kind of reaction may be seen under certain 

 circumstances. Thus, a fresh suspension mounted beneath a 

 raised cover will soon exhibit cloud effects due to differences in 

 the density of aggregation, and this corresponds to the first stage 

 of aggregation in Nereis. In the course of an hour or so all the 

 spermatozoa retract from the edges into a central dense aggrega- 

 tion, and this is due, I believe, to the rising CO 2 tension towards 

 the center. Reasons for this opinion are given under the head of 

 the aggregation phenomena. 



Fig. 4 Reaction of spermatozoa of Nereis to a drop of 1 per cent CO^ sea-water; 

 from an experiment of June IS, 1912. The original sperm suspension was made at 

 3.09 ; it aggregated clearly at 3.10 (fig. 1) . It was then mixed up with a pipette and 

 some drops mounted on a slide beneath a raised cover-slip as in 1. The drop of 

 1 per cent CO2 sea-water was introduced at 3.12| (on left) and a drop of pure sea- 

 water as control (drop to right). Figure 4, 1, shows the reaction at 3.13; 3, at 

 3.14; 3, at 3.14^, and 4, at 3.16. In 4 the general suspension has aggregated. The 

 final position of the reactive sperm is in the center of the introduced drop. No 

 reaction takes place with reference to the drop of sea-water, which gradually be- 

 comes obliterated by inwandering of sperm. The figure shows also that the sper- 

 matozoa retract from the margin of the suspension. 



III. AGGREGATION PHENOMENA 



The spermatozoa of Nereis and Arbacia show very definite 

 positive chemotaxis toward acids and egg-extracts of the same 

 species, which may be demonstrated with striking clearness by 

 the method first introduced by Jennings in studying the behavior 

 of Paramecium. The method as applied to behavior of spermato- 



