144 THE SCIENCE OF BIOLOGY 



somes to which the characters have been assigned, and hence 

 what might be described as a single dose of the character in 

 question. Union of the germ-cells to form the F x adult 

 produces an Aa combination in this pair of chromosomes. 

 The germ-cells of the Fi will be as shown, because reduction 

 of the chromosomes to one-half the adult number occurs 

 by disjunction of the members of the pairs. Combination 

 of these Fi germ-cells will occur in the 1:2:1 ratio, as indi- 

 cated by the F 2 adults of the figure, on the assumption of 

 their union by the laws of chance. 



Thus the arrangement and behavior of the chromosomes 

 is of such a nature as to explain segregation. The hypothe- 

 sis that the determiners for adult characters are carried by 

 the chromosomes is justified by the outcome. The chromo- 

 somes are distributed in a manner that parallels the heredity 

 of the members of a contrasting pair of unit characters. 



The foregoing explanation of Mendelian heredity by 

 means of chromosomes is an explanation in terms of cells. 

 Only, the analysis has gone deeper and disclosed the portion 

 of the cell that is primarily concerned. The diagnosis is more 

 complete than has been indicated here, giving consistent 

 results where additional pairs of characters are involved. 6 

 But the foregoing explanation is sufficient to show that the 

 mechanism of Mendelian heredity is a cellular one. Just as 

 the visible structure and functioning of the body are refer- 

 able to microscopic cells, so is the inheritance of structure 

 and function. Development consists of cell division and 

 differentiation from a single-celled organism, the oosperm, 

 to a many-celled organism the adult (Fig. 12). There is 

 cellular continuity between generations. Herein lies the 

 physical basis of heredity. It seems inconceiveable that a 

 single cell should contain within its limits an organization 

 capable of producing an adult if only a suitable environ- 

 ment is provided. But where adult characteristics are 



Cf. Table on p. 133 of the present volume, and the account of the chro- 

 mosome theory of sex determination on p. 204. 



