FIELDS AND GRADIENTS 3OI 



basalwards. In other cases, indifferent cells migrate in both 

 directions, but become progressively differentiated in different ways 

 according as they are moving apicalwards or basalwards. The 

 presence of a lateral graft, e.g. in Hydra, will induce a flow of cells 

 towards the graft; as is shown by heteroplastic experiments in 

 which the two types of tissue can be distinguished, some of these 

 cells grow out to form a base below the graft. 



There is thus a form of "dynamic determination" (see p. 163), 

 and although the graft acts in a way resembling an amphibian 

 organiser, it does so largely by a different method, namely, by 

 stimulating directive growth-processes. Something of the sort occurs 

 as part of the induction of new hydranths by stem grafts in 

 Corymorpha (p. 164), and in the case of grafted amphibian limbs 

 (p. 364). On the basis of these and numerous other experiments 

 he comes to conclusions rather different from those of Child. 

 However, although it is probable that the further analysis of 

 these directive movements of migration and growth will throw 

 much light on regeneration and differentiation, they cannot explain 

 a number of the facts previously cited in this chapter, for which 

 some form of field-theory is indispensable. 



(ix) Next we come to an extremely important rule, which is 

 that the action of external conditions upon gradient-fields and the 

 morphogenetic processes associated with them is always differential. 

 This appears to be a consequence of the quantitatively graded nature 

 of the fields. The differential action is revealed under three main, 

 heads, {a) differential inhibition, {b) differential stimulation or 

 acceleration, {c) differential acclimatisation and recovery. 



{a) When depressant agents are used in concentrations not 

 permitting acclimatisation, the most active regions are the most 

 susceptible, and suffer most. This is well seen when regenerating 

 Planarian fragments are exposed to narcotics. The heads regener- 

 ated under such conditions are not only subnormal in size, but 

 abnormal in form, in that certain regions are missing. The process 

 takes place progressively as the toxicity increases : first of all the 

 eyes become approximated, and then fused (absence of interocular 

 region) ; then the median part of the pre-ocular region fails to form. 

 Higher concentrations affect more lateral and more basal parts of 

 the head, until finally only a small basal head-rudiment, eyeless but 



