300 THE WHENOB AND THE WHITHER OF MAN 



new position, not the one for which they were normally 

 intended. And the organ which they would have 

 builded in a normal embryo was now formed by other 

 cells transferred to their rightful place. 



What made them thus change ? Not change of sub- 

 stance or structure, for the sHght pressure could hardly 

 have modified this. Not change of nutriment. The 

 only visible or easily conceivable change was in posi- 

 tion relative to other cells of the embryo. 



Let us in imagination simplify Driesch's experiment, 

 for the sake of gaining a clearer view of its meaning. 

 In a certain embryo at an early stage are certain cells 

 whose descendants should form the lining of the in- 

 testine and be used in the adult for digestion. A 

 second set of cells should form muscle endowed mainly 

 with contractility. When these two sets of cells, or 

 some of them, exchange positions in the embryo, they 

 exchange lines of development. The first set now 

 form muscle, the second digestive tissue. The only 

 change has been in their relative positions. Driesch 

 maintains, therefore, that the goal of development in 

 any embryonic cell is determined not by structure or 

 nutriment but by position. And this would seem to 

 be true of the cells of the earliest embryonic stages. 



Certain other experiments point in the same direc- 

 tion. Cut a hydra into equal halves and each half 

 will form a complete animal. The lower half forms a 

 new top, with mouth and tentacles ; the upper half, 

 a new base. Cut the other hydra a hair's-breadth 

 farther up. The same layer of cells which in the 

 first animal formed the lower exposed surface of the 

 upper half now forms the upper exposed surface of 

 the lower half. And with this change of position it 



