I THE PROBLEM 3 



animals, they are all comparable in that each is a distinct 

 and separate sexual cell which, as a rule, is unable to de- 

 velop 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 move- 

 ment. 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 semifluid medium (frequently within the body of the 

 female parent) before the necessary fusion can occur. • To 

 accomphsh this latter end of its journey the spermatozoon 

 is endowed with some form of motile apparatus, and th'is 

 frequently takes the form of a long flagellum, or whip-like 

 process, 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 or ovules are 

 larger than the pollen grains, though the disparity in size 

 is not nearly so marked. Still they are always relatively 

 minute cells since the circumstances of their develop- 

 ment as parasites upon the mother plant render it unnec- 

 essary 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 po- 



