2 MEXDELISM chap. 



servation of the minute sexual cells made possible. After 

 more than a hundred years of conflict lasting until the end 

 of the eighteenth century, scientific men settled down to 

 the view that each of the sexes makes a definite material 

 contribution to the offspring produced by their joint 

 efforts. Among animals the female contributes the ovum 

 and the male the spermatozoon ; among plants the cor- 

 responding cells are the ovules and pollen grains. 



As a general rule it may be stated that the reproductive 

 cells produced by the female are relatively large and 

 without the power of independent movement. In addi- 

 tion to the actual living substance which is to take part 

 in the formation of a new individual, the ova are more or 

 less heavily loaded with the yolk substance that is to pro- 

 vide for the nutrition of the developing embryo during the 

 early stages of its existence. The size of the ova varies 

 enormously in different animals. In birds and reptiles 

 where the contents of the egg form the sole resources of 

 the developing young they are very large in comparison 

 with the size of the animal which lays them. In mam- 

 mals, on the other hand, where the young are parasitic 

 upon the mother during the earlier stages of their growth, 

 the eggs are minute and only contain the small amount of 

 yolk that enables them to reach the stage at which they 

 develop the processes for attaching themselves to the wall 

 of the maternal uterus. But whatever the differences in 

 the size and appearance of the ova produced by different 



