FIELDS AND GRADIENTS 



287 



the formation of rudiments of distal and proximal tentacles of a 

 certain size, distance apart, and distance from the apical point. But 

 these values are smaller if the piece of stem is shorter^ (see also 

 p. 318). Similarly, when the Ascidian Clavellina undergoes de- 

 differentiation into a small mass of cells, and subsequently re- 

 differentiates into a well-proportioned Clavellina of reduced size,^ 



mill 



a 



III! 



I %i 

 I tv 



ill 



lilfl 



H '& 5- T' « 



m 



i 



Fig. 136 

 Modification of the scale of organisation in reconstitution in stem-pieces of 

 Tubularia. a, Future mouth region; b, primordia of apical tentacles; c, future 

 hypostome; d, primordia of main (basal) tentacles, i, Under standard condi- 

 tions. 2, In optimal conditions: the scale of organisation is enlarged. 3, In sub- 

 normal conditions : the scale is decreased. (From Child, Individuality in Organisms, 

 Chicago, 19 1 5.) 



one might say that the various fields are localised in terms of relative 

 quantitative positions along the main gradients : and these relations 

 holding for different total sizes, the control exerted by the dominant 

 region will be harmonic. 



As already mentioned (p. 165), the extent of the field dominated 

 by an apical region can be experimentally modified. Narcotics re- 

 duce the size of the regenerated head in pieces of Planarians ; the 

 size of the reconstituted pharynx, as also its distance from the 



Driesch, 1899; Child, 193 1 



- Huxley, 1926. 



