FIELDS AND GRADIENTS IN NORMAL ONTOGENY 325 



§ 4. Inhibition, physiological isolation, and multiple potentiality 

 of fields in ontogeny 



(vi) Inhibition exerted by a dominant region 

 on other parts of the system 



Perhaps the most striking example of this in early ontogeny is 

 found in sea-urchins. Here, the presence of the organiser (gastru- 

 lating) region inhibits the formation of long cilia on the late blastula 

 and gastrula, except for a small tuft at the apical pole. In the ab- 

 sence of the organiser, these cilia spread over all or most of the 

 surface of the blastula (see p. 103). The inhibition is here exerted 

 by the dominant region of the secondary or vegetative-animal 

 gradient (p. 320); but the principle is the same as in the example 

 given in the preceding chapter. 



No cases of resorption of a subordinate by a dominant region are 

 known in early embryology. The resorption of parts occurring at 

 metamorphosis (Amphibia, Echinodermata), and the partial re- 

 sorption of one member of a pair of double monsters by the other 

 are clearly of rather a different nature. However, an alteration in 

 relative size of parts can often be obtained as the result of differ- 

 ential inhibition. This is so in the experiments on Chaetopterus 

 larvae and Echinoid plutei, described on p. 332: it indicates that 

 there is a competition for available food-material between the 

 different parts of the embryo, and that the degree of success in that 

 competition is, in part at least, regulated by the relative activity of 

 the dominant region and other parts of the organism. 



(vii) Physiological isolation and the multiple 

 potentiality of gradient-field systems^ 



The fact that in many forms the early stages of development can 

 be made, by appropriate fragmentation, to produce more than one 

 normal larva was one of the earliest discoveries of the science of 

 experimental embryology. It attracted a great deal of attention, and 

 led Driesch to formulate his conception of ''harmonic equipo- 

 tential systems" (p. 353). 



Numerous examples of this have been given. We need only recall 



^ And see corollary, Chap, vin, p. 294. 



