148 INDUCTION AND ORGANISATION 



from a central maximum it shows a gradual decrease towards 

 the periphery. Therefore, a peripheral zone, in which the 

 potency is weak, gradually merges into material that lacks it 

 altogether. Moreover, the area in which the potency occurs is 

 initially much larger than the area from which the organ will 

 eventually develop. In the beginning, therefore, the organ 

 primordia occupy fairly large areas with vague boundaries. At 

 this stage, these areas may overlap at the edges, the cells in 

 the intermediate zones possessing two or more organ-forming 

 potencies simultaneously. In the course of development, the 

 potency becomes concentrated in the centre of the primordium. 

 The peripheral parts, in which the potency was weaker already, 

 now lose it altogether. Probably the ''physiological competition" 

 between the cells (p. 126) plays a role here. The result is a clear 

 demarcation of the organ primordia from their environment. 



The organ primordia, again, have the character of organ- 

 isation-fields. This is proved by the fact that they possess the 

 power of regulation, and that the organ-forming potency can 

 be transposed with respect to the material of the germ. In early 

 primordia, parts of the material of the field can be exchanged, 

 e.g. by inverting part of the primordium, without causing 

 disturbances in later development. A smaller, but harmoniously 

 built organ will develop from an isolated part of a primordium, 

 so that in experiments one primordium may give rise to several 

 organs (cf. our discussion of the limb primordium, p. 142). 

 Conversely, two or more equivalent primordia can fuse so 

 that they produce a single harmoniously built organ of double 

 size. In other words, the organisation-field may be divided into 

 several fields, and, on the other hand, two equivalent fields may 

 be combined into one unity. In each organ-field, therefore, we 

 find, on a smaller scale, a repetition of the regularities shown 

 by the organisation-field of the whole embryo at an earlier 

 stage in development (cf. p. 122). 



Initially, all cells of the field possess the same organ-forming 

 potency, though not all to the same degree. Later, however, 

 a further subdivision of the field takes place : the determination 

 of the parts of the organ. Under the influence of the field- 

 factors, chemodifferentiation takes place within the field, so 



