2 MENDELISM CHAP. 



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 con- 

 tributes the ,ovum and the male the spermatozoon ; 

 among plants the corresponding cells are the ovules 

 and pollen grains. 



As a general rule it may be stated that the re- 

 productive cells produced by the female are relatively 

 large and without the power of independent move- 

 ment. In addition 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 provide 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 mammals, 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 animals, they are all 

 comparable in that each is a distinct and separate 

 sexual cell which, as a rule, is unable to develop 



