FERTILISATION OF THE EGG8 OF ANlMALf^. 153 



egg- is less, is, then, in the eg'gs which have stood twenty- 

 four hours in water, slightly more than the length of the 

 spermatozoa (Fig. 1). 



The presence of tlie gelatinous coat is quite unessential to 

 the union of spermatozoa and eggs, for if by shaking it be 

 removed, fertilisation will still take place with the greatest 

 ease. 



AVhen liberated at the top, the eggs gradually sink to the 

 bottom of a beaker containing still sea-water. Tliey thus 

 appear to be heavier than their normal medium. Very small 

 currents are, however, sufficient to keep the eggs floating. 

 Probably in the sea, where the eggs are liberated, such 

 currents are always present. In tliat case, too, the currents 

 are of considerable importance in mixing the eggs and 

 spermatozoa. 



V. The Chemotactic Question. 



After repeated trials with unilateral illumination, I was 

 unable to detect the least sensitiveness of the spermatozoa of 

 Arbacia or Echinus for heliotactic stimuli. It was there- 

 fore not possible by application of such a stimulus to allow 

 the spermatozoa to stream in the direction of the eggs (which 

 may be done in the case of Fucus), and to observe whether, 

 when passing, they deviate toward them. 



A large number of artificial fertilisation experiments were 

 undertaken. To one side of a drop, either open or under a 

 raised coverglass, a small drop bearing spermatozoa was 

 added. The spermatozoa spread quickly in all directions, 

 and in the course of their wanderings came in contact with 

 the eggs, bringing about fertilisation. Within a few minutes ^ 

 this was made evident by the raising of the vitelline mem- 

 brane. I failed, however, to observe any attraction of the 

 spermatozoa toward the eggs from a distance, or any collec- 

 tion of the spermatozoa ai'ound the eggs outside the gela- 

 tinous coat. On the other hand, spermatozoa vrere Irequently 



