REGENERATION 



695 



cells older than cells of the earliest cleavage stages in development 

 have either undergone differentiation which is not completely re- 

 versible or else tissue culture media have not yet been developed 

 which permit the formation of proper organizers and fields of domi- 

 nance. 



When a group of cells is isolated from its dominating region, those 

 cells may regenerate a new body, as in the sponges, or only other 

 cells like themselves, as in tissue culture. Usually they lack a favor- 

 able environment, in the case of the more complex animals, and die. 

 The removal of the cell group reduces asymmetrically the remaining 

 portion of the body. 



When an integrated animal body is disturbed by a wound or by 

 removal of a part, certain conditions are set up under which healing 

 and sometimes more extensive regeneration take place. These are : 

 interference with or destruction of nervous control ; stagnation in the 

 transport system so that food and metabolic products accumulate; 

 change in diffusion gradients and the formation of new bio-electric 

 currents. The normal partition of foods is disturbed. Migration of 

 cells occurs to close the wound. Cells capable of growth assemble and 

 proliferate to reproduce the lost part under the influence of re- 

 maining local tissues. These cells are physiologically younger than 

 the differentiated cells of surrounding parts, utilize food faster, and 

 grow faster, so that regeneration takes place even when the organism 

 as a whole is starving. 



Dedifferentiation, when it occurs, is a sort of rejuvenation process 

 resulting in physiologically young cells with potencies comparable to 

 embryonic cells. 



ADAPTABILITY AND REGENERATION 



The capacity for minor repairs is exhibited universally by animal 

 organisms and is essential to their success. There is certainly no 

 close correlation, however, between regenerative capacity and suc- 

 cess of species. In general, regenerative capacity is a limiting fac- 

 tor to the individual, not to the species. In many cases, it seems to 

 be more economical from the viewpoint of the species to grow new 

 individuals by means of sexual reproduction than to repair dam- 

 aged ones by regeneration. Pishes and insects are highly successful 



