FORM REGULATION IN CERIANTHUS. 5 



sues during starvation supports this view. Now as regeneration 

 advances it is clear that in the smaller pieces the point where the 

 stimulus to regeneration is no longer sufficient to cause the 

 transfer of material or energy from the other tissues, would be 

 reached earlier than in larger where the total supply of available 

 material is much greater. Consequently regeneration in the 

 smaller piece is retarded earlier than in the larger, even though 

 the stimulus may be as great. The small piece is not neces- 

 sarily exhausted, for if a new cut is made it may regenerate 

 again, but the reserve supplies are held so closely by the other 

 tissues that the decreasing regenerative stimulus is insufficient to 

 render them available. A new cut surface means an increased 

 stimulus, and under these conditions material which was not 

 available during the later stages of the preceding regeneration 

 may now be made available by the increased stimulus, though in 

 this second regeneration the supply will run out sooner than 

 before. 



In the larger pieces of Ccriant/nis it is probable that the stim- 

 ulus to regeneration ceases, or reaches the level of normal rep- 

 arative stimuli, before a shortage of material occurs. A con- 

 venient designation for the condition of the smaller pieces is 

 " relative exhaustion." They are not absolutely exhausted, since 

 they may regenerate further under sufficiently powerful stimulus ; 

 they are, however, exhausted so far as the stimulus to which 

 they are subjected is concerned. There is nothing new in the 

 idea of relative exhaustion ; it is a well-known phenomenon in 

 biology and its role in regulative phenomena is undoubtedly im- 

 portant. 



It has been impossible to determine with any degree of exact- 

 ness the minimal size of pieces in which regeneration is possible. 

 In small pieces the inrolling of the cut margins becomes so irregu- 

 lar that the piece often takes a form in which normal regenera- 

 tion is impossible simply because of the relative position of parts. 

 Moreover, since the regenerative power is different at different 

 levels the minimal size of pieces differs according to the region of 

 the body from which they are taken. Short pieces in the 

 cesophageal region are not available for the aboral cut margin 

 of the oesophagus unites with the aboral cut margin of the body- 



