ii2 FERTILISATION [CH. 



secretion, such as is known to be produced by the archegonia 

 of Mosses and Ferns, or whether they meet the egg by 

 chance; it is usually assumed that the. egg exercises some 

 attractive influence, probably of a chemical nature.. It is 

 at least certain that the eggs of Echinoderms secrete a 

 substance which greatly increases the activity of the sper- 

 matozoa of the same species. It is also known that the 

 spermatozoon will enter a mature egg, or even a non- 

 nucleated fragment of one, in those species in which the 

 maturation divisions normally take place before the sper- 

 matozoon enters, while it will not penetrate into an egg 

 which is not yet ready to be fertilised. 



When a spermatozoon conies in contact with a naked egg, 

 and sometimes at least when it reaches the egg itself if an 

 outer covering is present, the surface of the egg-protoplasm 

 rises to meet it in the form of a small eminence known as 

 the reception cone (PL XII, A-E). The formation of this 

 conical projection of the egg-protoplasm is almost certainly 

 a surface-tension phenomenon, due to an alteration of the 

 surface tension at the point where the spermatozoon touches 

 the egg. The head of the spermatozoon is drawn into the 

 protoplasm of the reception cone, and in some cases the 

 tail is separated off; in others the whole spermatozoon 

 enters the egg. In Echinoderms immediately after the 

 spermatozoon touches the surface of the egg a delicate 

 membrane is formed from the egg surface (the so-called 

 vitelline or fertilisation membrane) ; its production begins 

 in the region immediately around the point of entrance of 

 the spermatozoon, but rapidly travels over the surface of 

 the egg, so that in an extremely short time the egg is 

 enclosed in a membrane -which is separated by a space 

 containing fluid from the surface of the egg. The forma- 

 tion of the fertilisation membrane of Echinoderms is also 



