184 READINGS IN BIOLOGICAL SCIENCE 



body wall, leaving the local epidermis in place in its normal location, then 

 brain and other axial organs will develop from this epidermis. In other 

 words, the center of organization can assert itself, and can induce belly- 

 epidermis, in its normal location, to form brain tissue. In fact, the center 

 of organization, when transplanted to a foreign site, can induce the forma- 

 tion of a whole new body axis, actually a little twin embryo attached to 

 one side of the primary embryo. 



Further research has shown that, rather than a single organizing center, 

 a series of such centers become operative as embryonic development pro- 

 gresses. The center just referred to, usually known as the primary or- 

 ganizer, is concerned with the establishment during normal development 

 of the main axial organs, such as brain, spinal cord, and associated struc- 

 tures. Other organization centers, the secondary organizers, have been 

 demonstrated which are concerned with development of parts of the eye, 

 the ear, mouth structures, and so on. And there are, in all probability, many 

 more secondary centers of organization which we know Uttle or nothing 

 about at the present time. 



It will be well to emphasize the particularly fundamental feature of 

 development that has been revealed by the methods of approach and the 

 new operative techniques just described. That fundamental feature of de- 

 velopment I shall call cellular interaction. Cells in a developing embryo 

 realize their own special roles in development as they are acted upon, and, 

 in turn, act upon other cells and groups of cells. 



Further evidence of interaction among cells has also been obtained from 

 a somewhat different type of research. My own interests in cellular inter- 

 action, together with those of several of my students, have centered for 

 a number of years around the phenomenon of regeneration. The word 

 regeneration, in the biological sense, refers to the replacement by cellular 

 multiplication and differentiation of structures lost by accident, or other- 

 wise removed from the body of an organism. It is well known that nearly 

 all animals possess the capacity for regeneration to a greater or lesser de- 

 gree. In higher vertebrates, such as man, the capacity for regeneration is 

 restricted, for the most part, to the formation of new tissue as it takes 

 place during wound healing. Certain lower vertebrates, however, nota- 

 bly the ampibia, possess a high degree of regeneration capacity. Entire 

 organs, such as a limb or a tail, can be completely regenerated. In a sense, 

 therefore, the abiUty to regenerate lost parts represents the retention of 

 the cells of an organism of embryonic capacities for growth and differen- 

 tiation. Thus, in the phenomenon of regeneration we have opportunity for 

 study and analysis of cellular interaction, as it underUes growth and dif- 

 ferentiation. 



If young animals are used, such as young salamanders, regeneration 

 proceeds with surprising rapidity. For example, if I take a young salaman- 

 der and amputate a fore limb across the upper arm, a new limb will grow 



