PHYSIOLOGY OF REGENERATION IN TUBULARIA. l6l 



also of all or most of the formative changes that follow. Irrita- 

 bility therefore is the essential factor in formative action and leads 

 first to change in position of the cells and ultimately to their dif- 

 ferentiation. Yet the process of closing gives every appearance 

 of taking place independently of the cells as units ; for the move- 

 ments appear to take place in the material as a whole and not to 

 be the sum total of a vast array of cell-adjustments. In other 

 words the cells adapt themselves as best they can to the mass 

 action that is going on and do not take the initiative in the 

 process. Nevertheless it is unsafe to argue from such gross 

 effects that cells have no independent functions. Their power to 

 divide at different times, and the cellular limits of differentiation, 

 show their independence in certain respects. If, however, as 

 seems probable, the cells are united by a meshwork of proto- 

 plasm the irritability of the living material may act without heed 

 to cell boundaries. 



The Nature of Formative Action. 



After the closure of the end of the stem of Tubularia the con- 

 dition is still one of unstable equilibrium ; by which I mean that 

 the stimuli received at the closed end cause further changes in 

 the relation of the end to the rest of the piece. The stimuli 

 may be entirely or largely internal, resulting from an 

 unstable termination. In Tubularia both external and internal 

 factors may act. This relation leads to the formation of a new 

 polyp in which the relation of the materials to the outside world 

 and to the rest of the piece is a stable one. If my analysis of 

 the factors involved, first in the closure of the stem, and later in 

 its further elaboration into a polyp, is correct we are led to the view 

 that the essential factor is the irritability of the material that deter- 

 mines the location of the formative changes. As I have given 

 recently (the Seventh International Congress, August, 1906) my 

 reasons for coming to this general conclusion I shall not enter 

 further into the argument here, but desire only to bring the inter- 

 pretation into connection with the present results. 



The modern school of developmental mechanics has sought 

 to explain formative actions as the result of familiar chemical 

 and physical phenomena but has not met with any marked 



