SEXUAL REPRODUCTION 223 



Certain seaweeds (Halidrys) have very large 



Fig. 25. Fertilisation of Halidrys siliquosa. I. Egg when 

 first extruded into the sea-water. II. Later stage, egg 

 spherical. III. Egg suddenly enlarged, just before 

 fertilisation. IV. Fertilisation. The sperm (S) has 

 slipped in at the " crinkled " spot (R). V. After ferti- 

 lisation the egg becomes crinkled all over and no longer 

 attractive, but poisonous to sperms. VI. The fertilised 

 egg has contracted and is surrounded by a newly formed 

 membrane. 



eggs, and if they are kept in sea-water as 

 they are extruded from the conceptacles, and 



