FORM REGULATION IN CERIANTHUS. 9 



These relations hold for all cases recorded, and, since the mar- 

 ginal tentacles correspond to the intermesenterial chambers, they 

 are exactly what might be expected. The number of marginal 

 tentacles which regenerate on a given piece is determined by the 

 number of intermesenterial chambers which it possesses, or the 

 number which it acquires by regeneration of mesenteries. The 

 arrangement of mesenteries in Ceriantlins was described in the 

 preceding paper. All mesenteries appear first at the oral end 

 and extend gradually in the aboral direction. In general, the 

 younger a mesentery the' less its length. Consequently as we 

 pass aborally the number of mesenteries actually present de- 

 creases. After section at any level where regeneration is possible 

 a powerful stimulus to growth exists ; the mesenteries which 

 extend into the piece remain and become united with the new 

 oesophagus and others regenerate. It appears that the power 

 to regenerate a given mesentery extends for a certain distance 

 aboral to the end of that mesentery and since the mesenteries 

 terminate at all levels we may now conclude that the " determi- 

 nation " of the body- wall for the regeneration of mesenteries shows 

 a similar arrangement. Thus the youngest, shortest mesenteries 

 can be regenerated only within a very short distance from the 

 oral end, the next older and longer within a somewhat greater 

 distance, and so on. The experimental results confirm and es- 

 tablish this conclusion in every case. The addition of new 

 material to the body of CeriantJius in producing increase in cir- 

 cumference and number of tentacles and mesenteries may be 

 conceived as a wedge of material which enters the body-wall at 

 the oral end and continually forces its way aborally, thus sepa- 

 rating the older parts of the body more and more widely. This 

 conception of the method of growth in the transverse direction 

 agrees with the view expressed above that the general reactive 

 capacity of the tissues is greatest at the oral end. The stimulus 

 to the formation of new mesenteries always becomes effective first 

 at the oral end and from here gradually extends aborally until it 

 is insufficient under ordinary conditions to give rise to a mesen- 

 tery. Even at this level, however, a mesentery may be formed 

 if the stimulus increases in intensity, or if the tissue becomes 

 more susceptible to it. The resulting conditions give rise to rapid 



