276 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



partners are adequate and exert a mutually balancing effect. As stated pre- 

 viously, there is reason for assuming that the normal contact mechanisms 

 depend at least partly on the interaction of adequate contact substances. 



The conditions prevailing at the point of junction may influence the occur- 

 rence or non-occurrence of regeneration in one of three ways: (1) The 

 presence of adequate contact mechanisms or contact substances may prevent 

 regeneration directly by insuring a relative state of rest; conversely, the 

 absence of such mechanisms or substances may directly cause regenerative 

 processes to set in; (2) the absence of adequate contact mechanisms may 

 lead primarily to the loosening of the connection between transplant and host, 

 and this may be followed by regeneration. In both of these cases we have 

 presumably to deal with specific actions of a chemical nature; (3) the ap- 

 proximation of the surfaces of contact may directly inhibit regeneration in a 

 simple mechanical way by exerting pressure. In addition, we have to consider 

 the growth momentum of both host tissue and transplant; the greater the 

 growth momentum, the greater must be the forces that tend to repress regen- 

 eration, other conditions being equal. 



While actual experience has proven the mutual antagonism between re- 

 generative activity and successful transplantation, other factors tend to make 

 regenerative processes favorable to transplantation. Thus a slight degree of 

 regenerative activity in many instances is needed for and makes possible the 

 joining together of host and transplant. There may exist, besides, an indirect 

 relation between the degree of transplantability and the degree of regenera- 

 tive activity which host and transplant exhibit; it depends upon the frequent 

 association of great regenerative power of organisms and their constituent 

 parts, with a primitive, less complex constitution and a correspondingly lower 

 degree of sensitiveness to differences in organismal differentials. There is 

 noticeable, therefore, particularly in phylogenetically and ontogenetically more 

 primitive organisms, a greater mutual adaptability between transplant and 

 host, and a greater ability of the transplant to withstand the injuries con- 

 nected with the process of grafting, especially during the first critical period 

 following transplantation when the nourishment of the grafts may as yet be 

 inadequate. But where the opposite conditions prevail, where there is a lack 

 of regenerative ability associated with a great sensitiveness of the tissues to 

 injuries, transplantation may be impossible, as, for example, in the case of 

 the adult mammalian ganglia cells of the central nervous system. 



It was presumably the difference in point of view between Yves Delage 

 and Giard which, more recently, suggested to Weiss the analysis of the factors 

 on which the antagonism between regeneration and transplantation depends. 

 In Salamander larvae, amputation of an extremity is followed by regeneration 

 of a new extremity; but if, according to Weiss, another extremity of such 

 a larva is transplanted onto the wound, regeneration is completely prevented, 

 provided the new extremity fits the defect anatomically as well as functionally ; 

 however, if the covering of the wound by the surface of the transplant is 

 incomplete, wound healing may take place at first, but then regeneration may 

 set in, and even if it is rudimentary or retarded, the transplant is cast off. 



