4. F. R. Lillie[71] has studied the very striking phenomenon of transitory sperm agglutination which takes place when the sperm of a sea urchin or of certain annelids is put into the supernatant sea water of eggs of the same species. If we put one or more drops of a very thick sperm suspension of the Californian sea urchin _S. purpuratus_ carefully into the centre of a dish containing 3 c.c. of ordinary sea water and let the drop stand for one-half to one minute and then by gentle agitation mix the sperm with the sea water the mass of thick sperm which is at first rather viscous is distributed equally in sea water in a few seconds and the result is a homogeneous sperm suspension. When, however, the same experiment is made with the sea water which has been standing for a short time over a large mass of eggs of the same species, the thick drop of sperm seems to be less miscible and instead of a homogeneous suspension we get, as a result, the formation of a large number of distinct clusters which are visible to the naked eye and which may possess a diameter of 1 or 2 mm. The rest of the sea water is almost free from sperm. These clusters of spermatozoa may last for from two to ten minutes and then dissolve by the gradual detachment of the spermatozoa from the periphery of the cluster. [71] Lillie, F. R., _Science_, 1913, xxxviii., 524; _Jour. Exper. Zoöl._, 1914, xvi., 523; _Biol. Bull._, 1915, xxviii., 18. This phenomenon seems to occur in sea urchins and annelids. The writer has vainly looked for it in different forms of the Californian starfish or molluscs and in fish at Woods Hole. Lillie failed to find it in the starfish at Woods Hole. The writer found that the sperm of the Californian sea urchin _Strongylocentrotus purpuratus_ will form clusters with the egg sea water of _purpuratus_ but not with that of _franciscanus_; while the sperm of _franciscanus_ will agglutinate with the egg sea water of both species, but the clusters last a little longer with the eggs of its own species. He also found that the clusters are more durable in a neutral than in a slightly alkaline solution and that the agglutination disappears the more rapidly the more alkaline the solution. The presence of bivalent cations, especially Ca, also favours the agglutination. It was also found that this agglutination occurs only when the spermatozoa are very motile; thus if a trace of KCN is added to a mass of thick sea-urchin sperm so that the spermatozoa become immotile a drop of this sperm will not agglutinate when put in egg sea water of the same species; while later, after the HCN has evaporated, the same sperm will agglutinate when put into such sea water.