THE FERTILISATION OF THE EGG 179 



results which do not support the conclusion that in general 

 the eggs of marine animals give off substances which actually 

 direct the movement of the sperm. Dakin and Fordham 

 themselves were unable to demonstrate chemotaxis in the sperm 

 of the mollusc Teredo. The drop method employed by 

 Lillie and his pupils yields results which are interesting but 

 somewhat difficult to bring into relation with those obtained 

 by Dakin and Fordham, though Lillie himself advocates 

 the existence of chemotaxis. When a drop of the egg- water 

 of Arbacia is introduced under a cover slip into a sperm- 

 suspension of the same species three effects are manifest 

 on microscopic examination. There is momentarily an 

 intensification of the normal motility of the sperm. This is 

 followed by an effect which has the appearance of a precipita- 

 tion and takes place in two stages : {a) aggregation, the loose 

 association of spermatozoa in groups which can be imitated 

 by passing CO2 into a sperm- suspension, and also occurs 

 spontaneously in dense sperm-suspensions presumably through 

 the accumulation of their own respiratory products ; 

 {h) agglutination, in which masses of sperm firmly adhere 

 together. This latter phenomenon is reversible, when the 

 egg-water is prepared from the same species as that from 

 which the open suspension is derived. That is to say, after 

 a few seconds or minutes, the sperm-masses separate, but 

 individually the sperms remain immobile. Similar phenomena 

 have been described in Nereis, Asterias, and Echinarachnius. 

 It is possible to study the agglutinating reaction 

 quantitatively by determining the greatest dilution at which 

 an indisputable reaction occurs for given samples of egg- water. 

 The properties of the agglutinating substance have been worked 

 out by LilUe and others, and it has been shown that the sub- 

 stance is not excreted by the fertilised but only by the 

 unfertilised egg. It appears to be of colloidal nature. The 

 egg- waters prepared from Arbacia and from Nereis both con- 

 tain substances capable of agglutinating the sperm of the same 

 species. The egg- water of Nereis produces no effect on the 

 sperm of Arbacia. On the other hand, sperm- suspensions of 

 Nereis undergo agglutination in presence of egg-water of 



