22 FRANK R. LILLIE. 



conditions under which it occurs, in response to egg secretions 

 of the same species, its character, reversibility, and the specificity 

 of the reaction are identical with Arhacia. It is apparently, 

 however, less pronounced, and therefore not so readily recog- 

 nizable of itself as an agglutination phenomenon. Even the 

 "apparent surface tension phenomena" which Loeb describes 

 for the clusters — "Short streaks or cylinders contract into 

 spherical masses, the above described clusters; and long cylinders 

 break up into a series of small clusters" — are the same as I 

 previously described for Arbacia (1913a, pp. 550-551). 



Loeb's interpretation of the "cluster-formation" as a possible 

 tropistic reaction confuses the two sets of phenomena — ^viz., 

 aggregation (a true tropistic phenomenon) and agglutination — 

 which sperm suspensions may exhibit to the egg-sea-water of its 

 own species. But the aggregation (tropism) can take place 

 only when there is a gradient from the secretion to the sperma- 

 tozoa. This is realized under the conditions of my experiment 

 of injecting a drop of egg-sea-water into a fresh sperm suspension 

 beneath a raised cover slip; in such a case the two phenomena 

 take place simultaneously viz.; aggregation in the form of a 

 ring around or in the introduced drop (depending on concentra- 

 tion), and agglutination. These two phenomena are produced 

 by two constituents of the egg-sea-water, as I have already 

 maintained. 



For the study of the aggregation phenomena therefore it is 

 desirable to employ an agent which has no agglutinative action. 

 This I did in an extensive series of experiments by the method 

 just referred to (i 913, p. 533 ff.). To illustrate :— a drop of a Vioo 

 dilution of a saturated solution of CO2 in sea-water injected into 

 a sperm suspension of Nereis in sea-water mounted beneath a 

 raised cover-slip is marked within a few seconds by the formation 

 of a ring of active spermatozoa within the margin of the intro- 

 duced drop, and separated from the general sperm suspension by 

 a clear zone nearly free of spermatozoa 1.5 to 2 mm. in diameter. 

 I interpreted the ring formation as a positive reaction to the 

 attractive substance (CO2 and acids generally) ; the spermatozoa 

 follow the gradient from the suspension into the drop containing 

 CO2 a certain distance, i. e., up to a certain concentration, and 



