190 The Unity of the Organism 



would not only continue to live, but would develop into com- 

 plete worms, the new alimentary tract being formed, as the 

 quotations show, from the internal tissues of the severed 

 piece ; that is, from tissues which in the normal worm have 

 nothing to do with the digestive organs. 



The demonstration of this ability of organisms to press 

 into service certain of their parts to replace other parts 

 that have been lost, even though the parts implicated are 

 normally quite different, structurally, functionally and de- 

 velopmentally, is undoubtedly one of the most important re- 

 sults of the researches on animal regeneration that were so 

 eagerly pursued a few years ago. A goodl}^ number of in- 

 stances of this in widely separated sections of the animal 

 kingdom have been established beyond cavil. 



Nusbaum has performed the useful office of summarizing 

 these on the basis of the kinds of tissues involved, as fol- 

 lows: 



"1. Formation of muscle elements from epithelial tissues 

 of ectodermal orioin. 



"'2. Formation of connective tissue elements from epi- 

 thelial tissue of ectodermal origin. 



'"3. Formation of muscle elements from differentiated 

 parenchyma cells of mesodermal origin (from connective 

 tissue). 



"■i. Formation of nerve elements from differentiated epi- 

 thelial tissues of mesodermal origin." ^^ 



Suitunari/ of Examination of Inadequacy of Cell-Theory 



We have now passed under review several large and quite 

 distinct groups of knowledge pertaining to those biotic ob- 

 jects called cells, all of this knowledge favoring the inter- 

 pretation of these bodies as differentiated parts or members 

 of the larg-er bodies to which thcv bcloufr. Thev come into 

 existence one after another as a consequence of the growth 



