204 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



commonly used in the case of higher organisms, in the more primitive ani- 

 mals, as a rule, larger parts are joined together. In some instances they are 

 so large that the procedure is comparable to parabiosis rather than to ordinary 

 tissue transplantation, except that in the typical parabiosis the size of the 

 area of union between the two partners is usually much smaller than in 

 transplantation as practiced in lower organisms. However, it is not only the 

 size of the pieces joined together which suggests a comparison with parabio- 

 sis, but also the fact that in invertebrates parts of organisms have, on the 

 whole, a much greater capacity to carry on an independent life and to restitute 

 the whole organism than the corresponding pieces in vertebrates. The pieces 

 to be joined together are therefore usually more independent of each other 

 and more self-sufficient than is the case in ordinary transplantation in higher 

 organisms. We might also express these differences by distinguishing be- 

 tween organismal transplantations in which organisms or parts of organisms 

 capable of independent life and of restitution into whole organisms are joined 

 together, and tissue or organ transplantations in which the transplants are 

 devoid of such capabilities. 



It is not our aim to survey the whole field of transplantations in inverte- 

 brates and lower vertebrate classes as such, but to use these experiments 

 merely as a means for the study of the organismal differences in their func- 

 tion of sustaining the tissue and organ equilibrium, and making thereby pos- 

 sible the maintenance of the individual organism. It is especially the experi- 

 ments on coelenterates and planarians of Jacques Loeb, T. H. and L. V. 

 Morgan, Wetzel, Peebles, H. D. King, E. N. Browne, Rand, Issayew, Child, 

 Goetsch, Burt, Mutz and Santos on which our conclusions are based. 



A. Organismal Differentials and Organ and Tissue Equilibria 



in Coelenterates 



In a general way it can be stated that two sets of factors determine in 

 coelenterates the kind of organ which is to be formed and its localization, 

 namely (1) a more or less rudimentary preformed differentiation of the 

 various parts of the body of an organism, and (2) the ability of parts of the 

 organism to undergo structural changes and to restitute a whole organism 

 from parts under varied conditions of the inner or outer environment. Instead 

 of the relative fixity in the structural relations between the various tissues 

 and organs which is characteristic of higher organisms, we find here a 

 primitive and very incomplete differentiation, associated with a great degree 

 of plasticity in the modes of response to altered conditions. It seems that 

 each part of the organism has a tendency to produce a certain area of the 

 organism or a certain organ system (pole) rather than another one, but often 

 this tendency can be overcome; moreover, the readiness with which organs, 

 other than those normally occurring in a given area, can be induced to form 

 by experimental means (heteromorphosis) differs in different parts of the 

 organism, the resistance being greater in those areas where the preformed 

 organization, rudimentary though it is, tends to the formation of a more or 

 less well differentiated organ area. 



