THE CELLULAR BASIS 163 



ent types of fertilized eggs or oosperms which 

 could be produced by a single pair of parents 

 would be (16,777,036) 2 , or approximately 

 three hundred thousand billions. But prob- 

 ably other things than chromosomes differ in 

 different germ cells, and it is by no means 

 certain that individual chromosomes are al- 

 ways composed of the same chromomeres, or 

 units of the next smaller order, and in view of 

 these possibilities it may well be that every hu- 

 man germ cell differs morphologically and 

 physiologically from every other one, in short 

 that every oosperm and every individual which 

 develops from it is absolutely unique. 



Indeed the production of unique individuals 

 seems to be the chief purpose and result of 

 sexual reproduction. In asexual reproduc- 

 tion the individual variations which occur are 

 chiefly if not entirely due to environment, 

 but in sexual reproduction they are also due to 

 new combinations of hereditary elements. 

 The particular germinal organization trans- 

 mitted from one generation to the next de- 

 pends upon (a) the, ancestral organization, 

 (b) the particular character of the cell di- 



