FRRI'ILISATION OF THE EGGS OF ANIMALS. 175 



soon as this comes in contact with it by the tip of its head. 

 On the other hand, it is possible, and even probable, that un- 

 known stimuli here play a part. The advance of the sptn-- 

 matozoon into the egg after leaving the periphery is, like the 

 formation of the vitelline membrane, doubtless due to a 

 stimulus given the egg by the spermatozoon. With regard 

 to the exact nature of the stimulus and of the protoplasmic 

 movements which appear to be its reaction we are as yet 

 without auy explanation. 



IX. Summary of the Chief Results. 



The chief conclusions arrived at during the research upon 

 the fertilisation of the eggs of the Echinoidea were as 

 follows : 



1. The meeting of the spermatozoa with the outer surface 

 of the gelatinous coat (zona pellucida) is a matter of chance, 

 and not due to chemotaxis. 



2. The passage of the spermatozoa through the gelatinous 

 coat (observed chiefly in Echinus) is more or less ina radial 

 direction as regards the egg. The direction taken is not due 

 to any chemotactic substance being excreted from the egg. 

 The phenomenon is possibly due to stereotaxis, but a purely 

 mechanical explanation seems to the author moi'e probable. 



3. The spermatozoa are probably not chemotactically 

 sensitive. They do not respond to tonotactic or heliotactic 

 stimuli, 



4. On coming in contact with a surface bounding their 

 medium the spermatozoa cling to it, and usually continue for a 

 time to revolve upon it in (from their point of view) a counter- 

 clockwise direction. This statement applies to every group 

 of the Echinodermata. 



5. The spermatozoa easily become attached to glass and 

 other surfaces by the tips of their conical heads. This 

 phenomenon doubtless plays a role in causing the spermatozoa 

 to bore through the gelatinous coat after having come in con- 



