222 PLANT LIFE 
near the tips of the fronds. The eggs and 
sperms are extruded from the conceptacles 
into the sea-water, and the sperms are soon 
observed to be actively swimming in all 
directions. At first the eggs exercise no 
influence upon them, but as the membranes, in 
which they are at first enveloped, dissolve in 
the water, the sperms are seen to cluster 
around the eggs, and each egg becomes the 
centre of a crowd of male gametes which 
are endeavouring to gain entrance into its 
substance. Presently one slips through the 
peripheral limiting pellicle of the protoplasm 
and gains the interior of the egg. It passes 
rapidly through the cytoplasm and becomes 
appressed to the egg-nucleus. In a few 
seconds it swells up, and finally the two 
nuclei, belonging to the egg and sperm 
respectively, coalesce, and fertilisation is thus 
achieved. 
Now it is a remarkable fact that during 
fertilisation only one sperm is required to 
fertilise the relatively large egg. This is true 
of animals as well as plants. Experiments 
have clearly proved that normally only one 
male cell can enter the egg at all, and that in 
any case only one male nucleus fuses with the 
egg nucleus. The study of seaweeds has 
furnished a clue to the means by which the 
entrance into the egg of but one of the crowd 
of struggling sperms is effected. It has also 
thrown light on some important features of 
fertilisation itself. 
