282 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



this function, but later on they lose it. In these instances there may be active 

 the secretion of an acid or of a proteolytic enzyme possessing the power of 

 dissolving the skin and serving as a contact substance. In a corresponding 

 manner, Helff has shown that it is due to absorption processes taking place 

 in the gills, which must be in direct contact with the integument, that histolysis 

 in the overlying integument is initiated. The histolytic influence of the atro- 

 phying gills increases at first as metamorphosis proceeds, reaching a maximum 

 just prior to the release of the forelimb; and then gradually it subsides as the 

 gills undergo the final stage of atrophy. 



As stated previously, we think it justifiable to transfer these conceptions, 

 derived from what has been observed under experimental conditions, espe- 

 cially those prevailing after transplantation, to the equilibrium, which exists 

 normally in an organism, between adjoining tissues; here contact substances, 

 in addition to hormones, presumably determine the tissue equilibrium, and 

 disturbances of this equilibrium may lead to extensive regenerative processes 

 in phylogenetically or ontogenetically more primitive organisms, and to simple 

 wound healing in the more differentiated organisms. Among these contact 

 substances the organismal as well as organ differentials may play a part, the 

 organismal differentials gaining in importance with increasing phylogenetic 

 and ontogenetic development. 



Certain kinds of transplantation in the more primitive classes of inverte- 

 brates similarly contribute to the understanding of the significance and origin 

 of regenerative processes at or near the point of junction of graft and host 

 and to the interpretation of the factors that maintain the tissue equilibrium 

 within the same individual. We have seen that organismal differentials are 

 of importance in this process, as are also correct axis orientation and polar 

 direction of joined parts. This is true especially in the case of the more primi- 

 tive invertebrates as well as of plants. If the cut surfaces do not fit each other 

 completely, a regenerative outgrowth may take place from an uncovered point. 

 Moreover, in vertebrates as well as in invertebrates, regenerative processes 

 may proceed not only directly from the free surfaces of injured organisms, 

 or from surfaces exposed after incompatible pieces have separated, but also 

 from totipotent cells which migrate to the exposed surfaces. Such observa- 

 tions have been made, for instance, in amphibia by Hellmich, and by Spek 

 and others in the ascidian Clavelina. In the latter case, under various condi- 

 tions leading to budding, certain totipotent cells are attracted from the deeper 

 tissues to that point of the body where the growth processes are to take place. 

 It may be assumed here, too, that certain substances rather than purely 

 mechanical factors direct the movement of these cells. It seems that the sep- 

 aration of the transplanted parts may in some cases constitute the primary 

 process, which subsequently is followed by regeneration ; but in other cases, 

 as we have previously pointed out, it is very probable that incompatibilities 

 between the joined pieces lead to regenerative processes, which are thus pri- 

 mary, and that these are followed only secondarily by a separation of the parts. 



Cell equilibria which depend upon contact influences exerted by adjoining 

 cells upon each other, determine whether one or more embryos shall develop 



