III STAGES IN AN ACT OF REGENERATION 60 1 



TABLE 5 



TIME-TABLE OF EVENTS IN A TYPICAL ACT OF EPIMORPHIC REGENERATION 



Stage 



Epidermal overgrowth (closure) completed 

 Demolition completed 

 Onset of blastema-formation 

 Dedifferentiation complete 

 Cell-proliferation maximal 

 Gross differentiation first visible 

 Histological differentiation first visible 

 Regeneration complete 



The work of Sengel ( 1 95 1 ) showing that it is possible to induce partial regener- 

 ates in planarians, consisting only of organs from the pharynx backwards, shows 

 that regeneration does not fail completely if the normal apex of the morphogenetic 

 gradient is absent and indeed the production of micro- and acephalic regenerates 

 by Child and others also illustrates this. Truncated anterior regenerates similarly 

 are not uncommon, particularly in hydroid coelenterates (Child, 1941). This 

 further stresses the qualitative effects of quantitative differences in morphogenetic 

 potential, since only a quantitative, and no qualitative, change results if the end 

 of a gradient is removed or suppressed. Amputation removes part of the intrinsic 

 gradients (Brondsted, 1955) in morphogenetic potential, but normally this is 

 regenerated in the process of material regeneration (Abeloos, 1932, p. 175). 



De Giorgi and Guyenot (1923) have shown that growth and differentiation 

 can be dissociated in a regenerate, just as they appear to be in tissue-cidture. 

 If the leg- or tail-blastema of a urodele is grafted in an "indifferent" region of the 

 body it grows to the size at which visible differentiation normally begins but does 

 not differentiate. If reamputated it regrows to the same stage and again does not 

 differentiate. 



Differential growth continues for a long time after differentiation, particularly 

 in whole-body regenerates of planarians (Dresden, 1940). This no doubt is mor- 

 phallactic change which always plays some part in whole-body regeneration and 

 is active later than the epimorphic component (Abeloos, 1932). In some instances, 

 particularly when regenerates or supernumerary growths are induced by grafting, 

 there may be extensive differential movements of parts (Goldsmith, 1940), due 

 largely to active resorption of tissues in one region and proliferation elsewhere 

 (Santos, 1 931; Miller, 1938). 



The stages of regeneration overlap in time, at any point, and for each stage 

 there is a spatio-temporal sequence along the regenerate (Table 4). Moreover 

 there is asynchrony also between epidermis and deeper tissues at any point. 

 Accurate time-tables for the process are not possible therefore. Again the relative 

 durations of the various stages probably differ considerably in different animals. 

 Table 5 gives average times from amputation to the main epochs in the process, 

 computed from available results on whole-body and limb-regenerates, in the in- 

 vertebrates and lower vertebrates. 



Literature p. 64^ 



