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. 



