180 The Unitii of the OrganisTn 



this is that regenerative processes so frequently involve con- 

 siderable masses of more or less differentiated tissues rather 

 than individual cells. A sort of mass action or mass per- 

 formance takes place with little or no regard to the indi- 

 vidualized activities of the constituent clementij, whatever 

 they may be, cells, nuclei, centrosomes, chromosomes or what 

 not. The most palpable instances of this mass performance 

 are afforded by those restorative processes in which cell 

 division plays no part, or but a subordinate part. These 

 processes are accomplished by the activity of cells already 

 in existence, by the re-disposing of old cells, rather than 

 by the production of new ones. Thus Rand describes how 

 the "cells of the earthworm epidermis are seen to execute 

 an extensive movement from their original position to an 

 adjoining surface not previously occupied by epidermis." ^ 

 This movement, Rand shows, is not a passive one due to 

 pressure or pull by extraneous forces, but is inherent in tlic 

 cells themselves, and "must be occasioned by an agency 

 external to the cell, namely, by some factor of the con- 

 ditions resulting from the injury." Rand looked carefully 

 for dividing cells in the region of the wound but could find 

 no indication whatever of these. To the mode of regenera- 

 tion, "in which a part is transformed directly into a new 

 organism, or part of an organism without proliferation at 

 the cut-surface," Morgan has given the name Trior phallaons, 

 and sets it over against regeneration accomplished through 

 proliferation, which he calls epimor pilosis.- All investi- 

 gators now recognize the importance of this distinction. 



While cells occupy only a retired place in much of both 

 the purely descriptive and the speculative writings on re- 

 generation, it would be wrong to infer that tlie broader 

 biological conceptions based on the facts of regeneration 

 have really ignored the cell-theory to tlie extent that at 

 first sight seems to be the case. As a matter of fact, it turns 

 out that in many of the discussions of regeneration which 



