THE CYCLE OF LIFE 381 



bearers or vehicles of the hereditary qualities. It is quite 

 safe to say that the chromosomes, along with other germ- 

 cell constituents, stand in some definite causal relation 

 to the adult characters. Now the remarkable fact is that, 

 while the quite immature germ-cells have the same number 

 (n) of chromosomes as the body-cells of the species under 

 consideration, the mature germ-cells have half that number 



( - ) . By a kind of cell-division (meiosis), which is normally 



restricted to this one point in the entire life- history, the 

 number of chromosomes is reduced to one half the normal 

 number. It follows that when the ripe spermatozoon 



and the ripe ovum each with^ chromosomes unite in 



fertilization, the normal number n is restored. If there 

 were not some reduction of this sort, the number of chromo- 

 somes would be doubled at each fertilization, which is 

 absurd. Moreover, in the reduction, which, in the case 

 of the egg, means the absolute rejection of half of the 

 chromosomes (which are usually carried off by the first 

 ' polar body ' and come to nothing), we see an opportunity 

 for permutations and combinations among the items of 

 the inheritance, e.g. for the dropping out of a character 

 altogether. If we compare the inheritance so far as it 

 is borne by the chromosomes to a pack of cards, there 

 is a remarkable throwing away of half of the pack and 

 their replacement by half of another pack at the beginning 

 of each individual life. 



The fourth wonder is fertilization the intimate and 

 orderly union of the reduced nuclei of the two sex- cells. 

 There are several processes involved which may be analysed 

 apart. There is the mingling of two inheritances, 



