284 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



the part of an organism and the environment may lead to growth processes 

 tending to the reduplication of the tissues. In such a finely equilibrated sys- 

 tem the normally present factors must cooperate to keep an organ, or a part 

 of an organism, at rest. Furthermore, a normal, non-transplanted part of an 

 organism which still tends to grow, may be induced by the presence of an 

 otherwise indifferent foreign body to produce an additional extremity, pro- 

 vided the necessary material for such an outgrowth is present. Or in very 

 primitive organisms, such as planarians, disturbance of the equilibrium by 

 mechanical means may lead to fargoing transformations in the individual, 

 and in coelenterates changes in the oxygen content in the surrounding me- 

 dium, or perhaps also diffusion of growth inhibiting substances out of the 

 animal may be followed by the formation of multiple growth centers. It may 

 be assumed that the contact with suitable tissues maintains an equilibrium in 

 which all parts of the organism are correlated in such a way that abnormal 

 growth processes are excluded ; distance substances also play a role in main- 

 taining this equilibrium. If these normal contact actions are interfered with, 

 outgrowths, which may lead to reduplication in some cases, take place in very 

 plastic material, while simple wound healing follows in higher, more differen- 

 tiated organisms. In all these instances the alteration in the environmental con- 

 dition represents the first link which sets in motion a chain of events leading 

 to the abnormal growth. It is of great interest to note the apparent similarity 

 in the initial factors, as well as in the subsequent links of the reaction chains, 

 which play a role in embryonal development, in budding, in the regenerative 

 newformation of organisms, and in the more simple wound healing as we 

 know it in higher organisms. 



Whether an outgrowth occurs from a tissue surface which is not adequately 

 covered by other tissue, depends also on the growth momentum inherent in the 

 substratum. The greater this momentum is, the greater the restraining action 

 of the transplant must be to become effective. The growth momentum is high- 

 est in the more primitive organisms. Here, too, transplantation of tissues suc- 

 ceeds better and differences in organismal differentials between host and 

 transplant play a less important part than in higher organisms. In the latter the 

 transplants have to overcome greater difficulties in holding their own, but they 

 have not to overcome as great a growth momentum in the host as do the trans- 

 plants in the more primitive organisms. Tissue transformers in the form of 

 organizers are lacking here, where the substratum has lost its plasticity. 



As to the character of the contact mechanisms, we have, as stated above, 

 presumably to deal with substances or chemical groups transmitted from one 

 surface to an adjoining one; conditions here seem to be analogous to those 

 observed in the case of the organizers, where effects exerted by chemical 

 substances are involved. Grafting experiments in embryonal and very primi- 

 tive adult organisms confirm and extend, therefore, our conceptions as to the 

 part which contact mechanisms play in higher and fully developed organisms. 

 There takes place a gradual transformation of the embryonal system of regu- 

 lation into the system of regulation of the higher adult organism, which, 

 because of the prominence of the organismal differentials, becomes an autog- 

 enous regulating system. This regulating system functions in higher organ- 



