284 DRIESCH'S THEORIES OF DEVELOPMENT V 



one of these is called into action ; this works on and alters the 

 cytoplasm; this then becomes receptive to a new and different 

 stimulus, and so on, as shown in the accompanying 1 diagram (Fig. 

 167). The cytoplasm thus becomes perpetually altered, while the 

 nucleus retains the entirety of the potentialities of the organism, 

 a supposition which is held in reserve to account for the phe- 

 nomena of budding and regeneration. 



The progressive limitation of potentialities, therefore, which 

 we observe in ontogeny is a limitation of sensibilities to stimuli, 

 equality of potentialities an equality of such sensibilities. In an 

 ' equipotential system ' (the blastula or the archenteron of 

 Echinus) all (meridional) parts are equally susceptible, and what 

 stimulus each will as a matter of fact receive is a function of its 

 position in the whole, that is to say of its distance from the 

 centres of force situate in the dissimilar points fixed ab initio in 

 the system of symmetry of the egg (Fig. 166). 



Development then proceeds from the comparatively simple 

 organization given in the structure of the fertilized ovum by the 

 creation of ever-increasing complexity by the action and reaction 

 of the parts on one another. Each ontogenetic effect produced 

 becomes in turn the cause of further effects, the possibility of 

 fresh specific action, for it becomes the seat of a new specific 

 stimulus and response, and so on until the complexity of the 

 ' ultimate organs ' of the adult is achieved. 



In order, however, that differentiation may be normal it is clear 

 that these stimuli and these responses must be accurately co- 

 ordinated : the right stimulus must be ready at the right time 

 and at the right place for the right organ to respond to. This 

 indispensable temporal and spatial co-ordination is the ( Causal 

 Harmony ' of development ; and it is given by the initial 

 structure of the egg and the constitution of the external environ- 

 ment. 



This is, however, not the only kind of harmony involved in 

 development. Potentialities become restricted as differentiation 

 progresses, and the development of the primary, secondary, 

 tertiary and subsequent systems of organs successively produced 

 is a process of ' self-differentiation ', depending on factors residing 

 in each system itself. Nevertheless these independent systems 



