I THE PROBLEM 3 



into a new individual of its species unless it is 

 fertilised by union with a sexual cell produced by 

 the male. 



The male sexual cells are always of microscopic 

 size and are produced in the generative gland or 

 testis in exceedingly large numbers. In addition 

 to their minuter size they differ from the ova in 

 their power of active movement. Animals present 

 various mechanisms by which the sexual elements 

 may be brought into juxtaposition, but in all cases 

 some distance must be traversed in. a fluid or semi- 

 fluid medium (frequently within the body of the 

 female parent) before the necessary fusion can 

 occur. To accomplish this latter end of its journey 

 the spermatozoon is endowed with some form of 

 motile apparatus, and this frequently takes the form 

 of a long flagellum, or whip-like pirocess, by the 

 lashing of which the little creature propels itself 

 much as a tadpole with its tail. 



In plants as in animals the female cells are 

 larger than the male cells, though the disparity 

 in size is not nearly so marked. Still they are 

 always' relatively minute since the circumstances of 

 their development as parasites upon the mother 

 plant render it unnecessary for them to possess any 

 great supply of food yolk. The ovules are found 

 surrounded by maternal tissue in the ovary, but 

 through the stigma and down the pistil a potential 

 passage is left for the male cell. The majority of 

 flowers are hermaphrodite, and in many cases they 

 are also self-fertilising. The anthers burst and the 

 contained pollen grains are then shed upon the 

 stigma. When this happens, the pollen cell slips 



