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 (8) 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 
