COELENTERATES AND PLANARIANS 207 



the organism which are not yet fully differentiated, to differentiate in such a 

 way that the dominating directing organ is supplemented and that a complete 

 organism develops. There are indications that the greatest potency of a certain 

 part is required if it is to function as an organizer and this is an attribute 

 mainly of the head, while apparently less potency is required for the regenera- 

 tive, integrative function of inducing supplementary organ formation and of 

 attaining in this way the formation of a whole organism by regenerative 

 means. Those parts which are functionally and structurally indifferent, such 

 as the middle piece, cannot act as organizers, nor can they induce regeneration. 

 The potential growth energy which is present in so marked a degree in these 

 primitive organisms, as exemplified in their response to inductive regenerative 

 influences and to organizer action, lies dormant in the normal organism ; the 

 mechanisms which cause induction of complementary parts and inhibit forma- 

 tion of similar parts are not ordinarily manifest. At each point the normal 

 contact mechanisms are active and keep the various parts in a quiescent state ; 

 but as soon as (through a cut or otherwise) this normal action of contact sub- 

 stances is disturbed, local growth processes set in, which are determined in 

 their character by a localized rudimentary differentiation, by regenerative 

 and integrative processes, such as induction and inhibition at a distance, by 

 organizer effects, and by environmental factors, all interacting with each 

 other. This interaction leads to the establishment of a new equilibrium which 

 takes the place of the previous disturbed equilibrium, and the most stable 

 equilibrium is reached when complete individuals are integrated. Within these 

 individuals the component parts are again equilibrized. 



However, a wound not only disturbs the regulating and inhibiting influences 

 which originate in the remaining parts of the organism and which would 

 normally act on the wounded area, but it exerts also a direct stimulating effect 

 on the tissues thus affected and its influence seems to extend even over a 

 relatively great distance, accelerating the formation of a hydranth on removal 

 of the inhibition existing normally. Through wound stimulation the organism 

 or parts of it are transformed in such a way that they resemble, in their be- 

 havior and reactions, organisms during the budding, reproductive state, when 

 they are very plastic and possess a greater growth momentum. We find here a 

 condition analogous to the autogenous regulating mechanisms which deter- 

 mine tissue equilibrium also in higher organisms and the kind of disturbance 

 in this equilibrium which follows injury. However, the relative importance of 

 the various factors which become potent following the making of a wound 

 cannot be exactly determined at the present time. 



Predifferentiation and coordinated integrative actions not only manifest 

 themselves through mechanisms which cause completion of incomplete or- 

 ganisms, but there must exist in addition, mechanisms which lead to degenera- 

 tion of excess tissues or organs ; we have referred to the resorption of small 

 parts which do not possess a pronounced differentiation. There may occur also 

 a coalescence of two small partners to form a single organism. 



We may then conclude from these data that what corresponds to the fixed 

 organ differentials of higher organisms, is, in the coelenterates, still in a very 



