272 ANIMAL AGGREGATIONS 



aggregations form as a definite positive reaction of the individual 

 spermatozoa to regions of self-produced higher concentrations of 

 carbon dioxide; that is, they are tropistic reactions. Cohn and 

 others believe that the aggregation is the result of a trap action 

 whereby the spermatozoa, on invading a region of higher CO2 con- 

 centration, have their activity reduced and tend to remain trapped. 

 We are not, at the present time, interested in the relative merits of 

 these two points of view. It is of interest, however, to note that, 

 according to both, the aggregations form without any evidence of 

 what we have called "secondary" or "social" reactions, but merely 

 as a result of the primary or individual reactions to the environment, 

 just as we have already seen that aggregations of animals frequently 

 form. It is also of interest that here with the spermatozoa, as in the 

 case of animal aggregations, far-reaching physiological effects may 

 result from these groupings. 



In the reaction of spermatozoa another aspect gives their behavior 

 more direct significance. Lillie and Just summarize evidence which 

 shows that among the other effects of egg secretions upon homotypic 

 sperm there is, at least in the sea urchin, a marked tendency to 

 aggregation in which "the spermatozoa are attracted to the drop of 

 egg secretion and gather in or around it, depending on its concen- 

 tration." Such reactions are not easily demonstrated, if they can be 

 demonstrated at all, by Pfeffer's capillary tube method, but can be 

 directly shown by introducing a drop of egg-water in a sperm sus- 

 pension on a slide covered by a raised cover slip. Under these con- 

 ditions, Lillie and Just report: "If a drop of egg water of Arbacia, 

 with agglutinating substance removed, be injected into a sperm 

 suspension of the same species, a ring of active spermatozoa forms 

 around the drop, separated by a clear zone almost devoid of sperma- 

 tozoa from the main suspension. If the clear zone be examined care- 

 fully, spermatozoa may be seen swimming directly across it from 

 the general suspension to the drop of egg water for some minutes. 

 The clear zone thus gives the range of some directive influence pro- 

 ceeding from the drop." This same phenomenon is exhibited clearly 

 by Nereis sperm when a drop of i per cent CO2 sea-water is similarly 

 introduced, and is illustrated in Figure 29 and explained in the 



