FIELDS AND GRADIENTS IN NORMAL ONTOGENY 353 



From the experiments in which isolated sea-urchin blastomeres 

 develop into perfect larvae, Driesch was led to formulate his 

 principle of "harmonic equipotential systems": equipotential 

 since parts can give rise to wholes and must therefore possess equal 

 and complete potencies : harmonic since the product is of normal 

 proportions and affords evidence of a definite relation-equilibrium 

 within the system. 



Driesch asserted that such systems afforded proof of vitalism. 

 We may however point out that the requirements of harmonic 

 equipotential systems are met by the theory of gradient-fields: 

 relative quantitative differences in activity-rate leading to qualitative 

 diflFerentiation : localisation being due to relative position along the 

 total length of a gradient. But it may be doubted whether true 

 harmonic equipotential systems have any existence in fact. The 

 1/4 blastomere of the sea-urchin regulates because it possesses the 

 whole extent of the gradient: divide it transversely, or, an even 

 more demonstrative case, divide the egg transversely (equatorially), 

 and no perfect larva will be formed (p. loi). The parts are not all 

 equipotential, although it may be possible, as in the case of blasto- 

 meres at the 4-cell stage, to eflFect subdivisions of a system without 

 segregating regions of different potencies. 



The limb-disc has been claimed to be a "harmonic equipotential 

 partial system ", but it does not appear that this connotation serves 

 any more useful purpose, or even carries the analysis as far as the 

 simpler concept of gradient-field, since, as already mentioned 

 (p. 223), limb-forming potency is unequally distributed round a 

 sub-central high point. 



The results of this chapter may be briefly summed up by saying 

 that in ontogeny the developing egg, prior to the stage of primary 

 chemo-differentiation, possesses an organisation in the shape of a 

 field-gradient system. The unitary and plastic nature of such a 

 system may be partly obscured by the unequal deposition of raw 

 materials, or by some degree of determination (though not an irre- 

 versible chemo-differentiation) having taken place before cleavage 

 begins. Further, matters are often complicated, notably in verte- 

 brates, by the existence of a second gradient- system connected with 

 the organiser. 



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