24 HEREDITY 



may be only tooVo^f '^^ t^® ovum's size. But, as 

 Thomson observes, " it may, however, be recalled (1) 

 that the physicists report that the image of a Great 

 Eastern filled, with framework as intricate as that of 

 the daintiest watch does not exaggerate the possi- 

 bilities of molecular complexity in a spermatozoon, 

 whose actual size may be less than the smallest dot 

 on the watch's face ; (2) that in development one step 

 conditions the next, and one structure often grows 

 out of another, so that we are not forced to stock the 

 microscopic germ-cells with more than initiatives." 



CHAPTER III 



THE FACTS AND FUNCTION OF SEX 



When we leave the lowest animals and plants and 

 consider the many-celled organisms we come upon 

 the fact of sex. We find that multicellular organisms 

 give rise to special reproductive cells, or gametes, one 

 kind of which is produced by female organisms and 

 another by male organisms. The former are called 

 macrogametes and the latter microgametes (i.e. large 

 and small gametes). The older terms applied to 

 these cells in the animal kingdom are ova and sper- 

 matozoa. These germ-cells, or gametes, are highly 

 typical and representative living cells, having the 

 structure already described. They, or rather their 

 nuclei, form " the material basis of inheritance " ; 

 and it is to them that the scientific study of heredity 

 constantly refers us. The first question we must ask 

 ourselves is as to the origin of these germ-cells. 



