WAYS OF ATTACKING THE PROBLEM 479 



In the rabbit it is possible, according to Russo, to distinguish 

 two kinds of ova in the ovary. According to Riddle there are 



FIG. 45. Decorative male and less adorned female of Spathura a genus 

 of Humming-brids. (From Darwin, after Brehm.) 



male-producing and female-producing ova in pigeons, differing in 

 their regime of metabolism. Dimorphism of spermatozoa is not 

 uncommon. In some cases the size-dimorphism of sperms is asso- 



