168 The Unity of the Organism 



What, on cursory view, looks more like confirmation of 

 the theory of cells as wholly independent elements whose 

 cooperation explains the organism, than any facts recently 

 brought to light, are some of the results of recent researches 

 on isolated tissues "tissue cultures," as they are frequently 

 called. The German phrase, ueberlebende Gewebe, sur- 

 viving tissues for these is very apt. 



Although the work of Alexis Carrel in this realm has at- 

 tracted much interest and attention even on the part of 

 the general public, it is recognized by biologists, including 

 Dr. Carrel, that R. G. Harrison not only initiated the 

 methods employed, but also reached highly important results 

 by applying them. The specific purposes and results of 

 Harrison's researches, and his general attitude toward prob- 

 lems of individual development, are of prime moment for 

 our discussion. His central aim was to end the perennial 

 debate over the mode of origin of nerve fibers in vertebrate 

 embryos, and the persistence, technical skill, and cogency 

 of reasoning with which he worked at the problem until he 

 advanced its solution sharply beyond the point reached by 

 any one else, are admirable. 



According to the view attributed to Wilhelm His, the 

 axis cylinders of the nerve fibers of the central nervous 

 system are outgrowths of the ganglionic cells, their con- 

 nection with the end organs being secondary. The other 

 view, less generally held, originally advanced by the physi- 

 ologist Victor Hensen, is that the fibers are differentiations 

 within protoplasmic strands which have connected the cen- 

 tral and peripheral cells from the very time when the cells 

 themselves were formed, their character as nerve fibers being 

 taken on only when, through the activities of the growing 

 embryo, conducting paths are needed. 



Harrison's earlier attempts to terminate the controversy 

 by transplanting limb-buds, under various conditions, from 

 one frog tadpole to another, had led him to believe strongly 



