REGENERATION 693 



Reduction of potency of individual cells of the adult organism is 

 really only another way of stating reduction of regenerative capacity. 

 As illustration of this parallel, it has been shown that a few isolated 

 cells from some of the sponges, kept in a proper environment, will 

 regenerate the whole organism. In contrast to the formation of an 

 entire organism by each of the first two blastomeres, if a few cells 

 from an adult mammal are isolated in a favorable environment, as in 

 tissue culture, the cells will only reproduce cells of their own kind, 

 not a new organism. Of course, some cells, such as nerve cells, will 

 not even reproduce their own kind and thus loss of reproductive or 

 regenerative potency is complete. 



Why does the position of a particular cell, in relation to other cells, 

 determine its subsequent history? 



Every animal organism exhibits polarity. The frog's egg is a 

 familiar example with its animal and vegetal pole; the nerve cell 

 with its afferent axone and its efferent dendrites is another. Polarity 

 may be determined in many ways; in the frog's egg by the relation 

 of the egg to its nutritive supply, in the nerve cell by the orientation 

 of the cell with respect to nervous center and periphery or to other 

 parts of the central nervous system. Polarity implies the presence of 

 a gradient of some sort between the poles — whether a gradient of ma- 

 terials as in the frog 's egg, or of permeability, of rate of metabolism, 

 or of some other sort. Usually the region at the more rapidly func- 

 tioning end of such a gradient dominates at least a portion of the 

 gradient. 



Dominance of one region over another may be exerted in various 

 ways. Thus the central nervous system dominates a major portion 

 of man's body by assorting stimuli from the various portions of the 

 body and sending impulses to the proper effectors. The endoerines 

 respond to other stimuli and send substances through the blood 

 stream. In other cases, substances are produced locally and produce 

 their characteristic effects after diffusion to nearby cells. Bio-electric 

 currents may also serve as an agency in the exercise of dominance. 

 Polarity, with its resulting gradients, dominance of the more active 

 region of the gradient over the less active, integrate the developing 

 organism. "When a region grows sufficiently large, a formerly sub- 

 ordinate level may become so far removed that it escapes from the 

 dominance of the more active region and itself becomes locaUy domi- 

 nant, resulting in agamic reproduction in the simpler forms, or in 



