Section XIII 



CHAPTER 2 



Vertebrates 



J. S. NICHOLAS 



INTRODUCTION 



Regeneration as defined by Nussbaum 

 (1886) is limited to the replacement of lost 

 parts by an individual during either its lar- 

 val or its adult existence. This would mean 

 that a structure must have completed its 

 morphological differentiation in order that 

 we may speak of its replacement as regener- 

 ation; otherwise it is replaced by the organ- 

 ization of an undifferentiated cell mass, the 

 cells of which are, in a non-specialized sense, 

 formative and, in another, equipotential in 

 the capacity for forming parts which are quite 

 different from their original prospective sig- 

 nificance. When one arbitrarily states that 

 an embryo has no capacity for regeneration, 

 as the definition above would indicate, it 

 is simply an expression of a limitation to 

 which we cannot now rigidly adhere. An 

 undetermined embryonic structure does not 

 regenerate because its prospective signifi- 

 cance has not yet been realized; it therefore 

 retains its prospective potency for the pro- 

 duction of structures with which it would 

 ordinarily not be concerned. A restatement 

 of the original usage of the term regenera- 

 tion would imply simply that regeneration 

 is the replacement of lost parts, no matter 

 when in the history of the individual this 

 occurs. Such usage would make unnecessary 

 the various loosely used terms such as res- 

 titution, reconstitution, replacement and reg- 

 ulation, as well as those peculiarly signifi- 

 cant to undetermined embryonic structures 

 such as super-regeneration or post-regenera- 

 tion. 



It is evident from this short discussion that 

 a definition of regeneration might be laid 

 down arbitrarily and therefore exactly, but 

 qualifications of the definition increase in 

 direct proportion to the study of the organ- 

 ism in its various phases. There is a con- 

 tinuity of the regenerative process — the 

 discontinuous elements are postulates not 

 actual. 



With this limitation in mind it is clear 

 that there must be a continuous overlap 



between the processes to which we refer as 

 embryonic, differentiative and regenerative. 

 They may be considered essentially as parts 

 of the continuum which is constantly present 

 as a potential process and which is most 

 significantly evident in those organisms which 

 completely replace parts lost by accident 

 and design; they also are less spectacular 

 but probably more important in the gen- 

 eralized process of growth and repair. The 

 first we might catalogue as reconstitutive 

 regeneration, the second as physiological 

 regeneration. These in turn overlap as proc- 

 esses within the organism; e.g., the pan- 

 creas, a doubly secreting structure, shows 

 both types of regeneration. The multiple 

 acini of the ducts are constantly changing 

 both in their cellular secretion and in the 

 cells themselves. In the ductless part of the 

 pancreas there is total destruction of huge 

 areas of islet cells, a period of quiescence 

 and later reestablishment of function. 



In general, the entire process might be 

 said to be limited, so far as reconstitutive 

 regeneration is concerned, to active replace- 

 ment of peripheral parts, either on the out- 

 side of the body where more than one tissue 

 is usually concerned, or internally where 

 a part of an organ is reconstituted. The 

 physiological regenerative process is much 

 more continuous in the vertebrates than is 

 the reconstitutive, and quantitatively ac- 

 counts for the immediate repair of large 

 areas of tissue reconstituted after their reg- 

 ular and sometimes cyclic loss. 



In addition to a high degree of physio- 

 logical regeneration, vertebrates possess ad- 

 ditional regulative mechanisms in a physio- 

 logical adaptation, accompanied, of course, 

 by morphological changes, termed "compen- 

 satory regulation." Under this heading come 

 hypertrophy and hyperplasia, which when 

 combined affect the regulation of other 

 portions of the system. 



In addition to the compensatory regula- 

 tory processes, there is constant replacement 

 of integumentary parts in nearly all forms. 

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