1 86 READINGS IN BIOLOGICAL SCIENCE 



nique, it has been possible to render and to maintain the limb of a small sal- 

 amander totally nerveless and thus completely to prohibit regeneration. 

 One of the results of these experiments has been that, when a limb fails 

 to regenerate, because of the lack of innervation, antagonistic processes 

 set in which lead to regression. The situation can best be presented by 

 describing a single experiment, 



A limb, rendered nerveless by previous operation, was amputated near 

 the wrist. Under these conditions no regeneration ensued, but on the 

 contrary, all of the skeletal elements, muscles, and other tissues of the wrist 

 went into regression and gradually disappeared. Then the regressive proc- 

 esses spread to the loM^er arm, and it, in turn, disappeared. Finally, the 

 tissues of the upper arm underwent the same changes. The processes in- 

 volved in these alterations are the opposite of growth and differentiation. 

 They are regression and dedifferentiation and represent, in a sense, em- 

 bryonic processes in reverse. It is as though a limb could say: "If I can't 

 regenerate, I insist on going into regression." 



To complete the account of this experiment, it must also be stated that, 

 if nerves are permitted to grow back into a limb at any time during the 

 process of regression, then the tables are turned, and growth and differen- 

 tiation begin at once in the limb tissues. There seems to be a balance be- 

 tween growth and differentiation on the one hand, and regression and de- 

 differentiation on the other, and it appears that the nerves of the limb in 

 some manner control this balance. 



I have now presented evidence from two different sources; first, from 

 the manner in which organs are established in early embryonic develop- 

 ment; second, from the manner in which tissues can be regenerated. These 

 two types of evidence have been chosen to illustrate the primary point, 

 namely, the cellular interaction is always in evidence and apparently of 

 paramount importance in the organization, differentiation, and growth 

 of structures. 



In conclusion, it should be emphasized that, regardless of how far mod- 

 ern biologists have traveled beyond seventeenth century workers in im- 

 proving their acquaintance with problems of organization and growth, 

 a woeful lack of real understanding of the processes still remains. It is 

 evident, I believe, to all experimental embryologists that, underlying the 

 organization which they study in embryonic and regenerating tissues, is 

 a physico-chemical background. It is important to keep in mind at all 

 times that there is a physiological chemistry of the developing embryo, 

 as well as of the adult body. When we develop conceptions of cellular 

 interaction during embryonic development, we are dealing, I am con- 

 vinced, with problems of inter- and intra-cellular chemistry. And it seems 

 certain that investigations of cellular chemistry, as related to growth and 

 differentiation, offer a tremendous field for future work. It is a field, how- 

 ever, in which successful investigation will continue to require the devis- 



