670 



Regeneration 



entiation results in the formation of excre- 

 tory products such as carbon dioxide and 

 nitrogenous compounds and these substances 

 have inhibitory effects. Therefore any two 

 groups of cells potentially able to regenerate 

 may be competing with each other by uti- 



ganism or by an environmental difference 

 imposed from without. The situation is out- 

 lined in Figure 230. 



Numerous simple experiments are consist- 

 ent with the competition hypothesis. In 

 Tubularia it is possible to select pieces of 



CONTROL OF REGENERATION BY INHIBITION 



By contact 



Inhibitor accumulates at surface of contact and hydranth develops 

 at free surface. 



By mutual intiibition 



Zooid 



Inhibitor accumulates in center, C, where four sources are in close 

 proximity. 



By COp 



7 

 Stolon 



C02,urea, uric acid permit stolon growth but inhibit zooid formation. 



Fig. 231. 



lizing common substrates and by excreting 

 common inhibitory substances. Now, if the 

 two groups of cells, A and B, differ initially 

 by some quantitative factor in their chemical 

 make-up, then A may be better equipped to 

 utilize substrates and resist inhibitory com- 

 pounds as compared with B. Therefore, A 

 may inhibit the regeneration of B and differ- 

 ent structures will result at A and B. The 

 initial difference between A and B would 

 be derived from a gradient in the whole or- 



stem of such a length and age that regenera- 

 tion always occurs at the distal (apical) 

 end but not at the proximal (basal) end. 

 It is only necessary to make a ligature at 

 the middle of such stems and then both 

 ends form hydranths. What are the effects 

 of tying a ligature? The tissue is constricted 

 and separated at the ligature so that circu- 

 lation between the two ends is stopped. 

 Neither inhibitory substances nor substrates 

 can pass from the apical end to the basal 



