FORM-REGULATION IN CERIANTHUS. HQ 



should prove to be the case the question as to polarity or struc- 

 tural conditions as the cause of the difference of time in the ap- 

 pearance of tentacles at the two ends would remain to be de- 

 cided. In the oesophageal pieces, however, the structure and 

 relations of mesenteries at the two ends are essentially the same, 

 no more modification being necessary at one end than at the 

 other to produce typical structural conditions for tentacle-forma- 

 tion. Since this is the case the absence of tentacles on the aboral 

 ends of oesophageal pieces seems at present to indicate that the 

 difference between the two ends as regards tentacle-formation 

 consists in something else than gross structural relations of parts, 

 i. <?., that some kind of polarity exists. 



The nature of organic polarity is at present exceedingly ob- 

 scure, but it is well known that the polarity of pieces as regards 

 regeneration is dependent upon the previous relations of the 

 pieces as parts of a whole. In pieces of the lower forms, such as 

 Hydra, other hydroids, Planaria, etc., the polarity may not be 

 manifest in any distinct structural differentiation at the two ends, 

 but merely in their functional condition. The piece, retaining 

 more or less completely the functional capacities of the whole 

 from which it was taken, uses, or attempts to use, its parts in a 

 similar manner, i. c., the piece, in continuing its functional life, 

 attempts in some degree to use its aboral or posterior end as the 

 aboral or posterior end of the whole is used, and the same is true 

 of its anterior end. The functional stimuli affecting these parts 

 are similar to, though perhaps less powerful than those in the 

 whole. It is probable that the regeneration of the characteristic 

 structures at the two ends is closely correlated with these stimuli. 

 When we can alter the conditions so that the aboral or posterior 

 end is affected by external stimuli which typically affect the oral 

 or anterior end while the " internal polarity " remains the same, 

 the result depends upon the relative value of external and internal 

 conditions as formative factors. Frequently in such cases the 

 regeneration is delayed ; apparently because of the " confusion ' : 

 of stimuli, /. e., the original stimuli and those resulting from the 

 atypical orientation. Finally one or the other dominates and the 

 "polarity" either remains as it was originally, or is reversed. 

 In more highly organized forms, where internal factors are much 



