FORM REGULATION IN CERIANTHUS. 2/5 



the determination of localized structures at certain points. In 

 each intermesenterial chamber the circulatory current passing 

 orally along the inner surface of the body-v/all (Fig. 2) must 

 strike against any part of the wall which is folded or rolled over 

 in such a manner as to form an obstacle in its course. When a 

 transverse cut in the body-wall of CcriantJms is made the cut 

 edges roll inward in such a manner that the current passing 

 orally in the chambers must strike the inner surface of the in- 

 rolled portion. In this condition that portion of each intermes- 

 enterial chamber just beneath the inrolled cut margin forms a 

 blind sac into which water is continually being forced by the 

 current. The cilia which in the normal animal provide for a 

 return current have been in large part removed from this region. 

 Local pressure upon the body- wall in this region must result. 

 Now the marginal tentacles always make their appearance in 

 exactly this region, a region which previously formed a part of 

 the lateral body-wall, and which, since tentacle-regeneration is 

 possible at any level except the extreme aboral region, cannot 

 have possessed any special qualifications for tentacle formation. 

 The question is, what causes tentacles to appear in this particular 

 region of a piece, no matter what part of the body the original 

 piece represents. Doubtless very few will regard the assump- 

 tion of an " entelechy " (Driesch) or of "dominants" (Reinke) 

 as an answer to this and similar questions, although it may 

 possess the merit of many other unwarranted assumptions, viz., 

 that of being an easy way out of difficulty. If we call in " hered- 

 ity " to answer the question we still have no real answer, though 

 it is evident that tentacles of characteristic form and structure 

 would not appear if CcriantJins were not concerned. Are we 

 not justified in seeking for definite, intelligible causes or condi- 

 tions of one kind or another for phenomena of this nature ? 

 Much work of recent years has given an affirmative answer to 

 this question. In many cases undoubtedly these causes or con- 

 ditions are internal, /. e., in the protoplasm, but it is by no means 

 impossible or improbable that they are external in many other 

 cases. 



Certain of my experimental results seem to me not only to 

 admit the possibility of local pressure as a determinative factor 



