FORM REGULATION IN CERIANTHUS. 2/3 



terial margins and mesenterial filaments cannot be determined. 

 Moreover, these are merely matters of detail and do not affect 

 the general outlines of the process. Since the position of the 

 organs is subject to considerable variation the direction of local 

 currents produced by their cilia must vary likewise. 



This discussion of the internal circulation and pressure has 

 been somewhat detailed, since I believe that the general internal 

 pressure and perhaps also local pressure resulting from the 

 impact of currents against the body-wall are important factors 

 in form-regulation and without doubt also in development. 



INTERNAL PRESSURE, GROWTH AND FORM-REGULATION-- A 



PRELIMINARY SURVEY. 



It is necessary before proceeding to the account of my experi- 

 ments to indicate briefly how growth and form-regulation may 

 be affected by the internal water-pressure. It would be difficult 

 otherwise in many cases to show the bearing of the experiments 

 without extended explanation. 



It is conceivable that the presence of water in the enteron may 

 affect the body-wall in two ways : first, by general pressure, the 

 same in all parts of the body, which subjects the body-wall to 

 tension ; second, by local pressure resulting from the impact of 

 definitely directed currents upon some portion of the body-wall 

 which interrupts their course, thus subjecting that part of the 

 body-wall to a localized tension in addition to the general tension. 



The experiments to be described demonstrate clearly that 

 regulation in Ccriantlnts is dependent in certain important respects 

 upon the tension resulting from internal water-pressure. The 

 tissues react to this tension by growth. In the absence of the 

 tension the typical form of the animal does not appear. Up to 

 this point the evidence afforded by the experiments can scarcely 

 be doubted or refuted. 



An essential feature in form-regulation in Ceriantlnis, as well 

 as in other forms, is localized growth which is especially notice- 

 able in the formation of new tentacles. 



The problem of the cause of localized growth is one of the 

 greatest importance in morphogenesis. According to one view 

 the basis of morphological form is inherent in the organism, 



