CHAPTER XXXIV 



REGENERATION 



(By T. C. Byerly, Poultry Research Division, United States 

 Department of Agriculture) 



INTRODUCTION 



Regeneration may be defined as the formation of new tissue to 

 replace that which has been lost. The fact of regeneration was 

 familiar to Aristotle and Pliny but Spallanzani, who published the 

 results of his researches in 1768, was the first to apply the experi- 

 mental method extensively to the study of the problem. Examples 

 of regenerative ability have been given in previous descriptions of 

 the various animal phyla. In general, the ability to regenerate lost 

 parts is more extensive in the simpler animal organisms and more 

 restricted in animals of great complexity. In every case, regenera- 

 tion involves the loss of tissue followed by reorganization of the 

 remaining tissues. Reorganization is usually followed by growth 

 of the regenerating portion until it reaches approximately normal 

 size in proportion to the total size of the body. Discussion of 

 regeneration will include consideration of the control by the organ- 

 ism of its parts and of the integration of the parts. 



REGENERATIVE CAPACITY 



Protozoa 



Regenerative capacity varies inversely with complexity of the or- 

 ganism but also varies a great deal among animals belonging to the 

 same phylum. Among the protozoa, nucleated fragments are gen- 

 erally capable of regeneration while nonnucleated fragments are 

 not. Thus, if Stentor is cut into three transverse pieces, each con- 

 taining a portion of the chainlike nucleus, each fragment regen- 

 erates by rearrangement of material, forming a small Stentor which 

 subsequently grows to normal size. In forms such as Amoeba, only 

 one fragment produced by cutting is likely to be nucleated. This 

 fragment will regenerate while nonnucleated fragments will round 

 up but fail to regenerate further. 



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