THE GENESIS OF LIVING ORGANISMS 213 



corresponds with the reduction in material, and into which 

 the head of the normally developed thigh-bone does not fit. 



From this we may conclude that the individual cells of 

 the germinal area are not of necessity destined to form special 

 parts of the bone, but that other cells, which in the ordinary 

 course of things would occupy other positions, are able to take 

 their place. Driesch, in his experiments on the whole germ, 

 draws attention to this fact, and from it deduces the concept 

 of the " harmonious and equipotential system," in which each 

 part can take the place of every other part without disturbing 

 the harmony of the whole. 



Moreover this fact proves that each individual cell does not 

 create its own special impulse directing it to a predestined 

 place, but that all the cells within a germinal area are directed 

 in common, behaving like iron-filings under the influence of a 

 magnet. 



To continue this analogy — a definite polar system of 

 " directives " acting in space invades the germinal area, and, 

 according to the amount of material present, produces a larger 

 or a smaller bodily structure, and from this, by release of 

 the bone-forming genes in the individual cells, there subse- 

 quently arises the complete bone. The existence of direction- 

 points has recently been demonstrated by Spemann in his 

 remarkable series of implantation experiments. But he calls 

 them " organisers," because they have the power to impose 

 their organisation upon the still undifferentiated cell-material 

 surrounding them. 



Since at a given time the same relations hold within each 

 germinal area, we can break up the whole embryo into a 

 number of independent polar systems of direction-points, each 

 distributing its own directives. But as Braus' experiment 

 proves, no system is able to exert an influence on the direc- 

 tives of another system, although the work of the directives 

 proceeds everywhere at a uniform rate. 



