FORM-REGULATION IN CERIANTHUS. 1 03 



sure persists either retain their form and size or regenerate as 

 the case may be. 



The atrophy of the tentacles above the cut in series 10 is very 

 clearly due to the same factor which caused retardation in 

 regeneration and later reduction and atrophy of the tentacles 

 over the cut in series 42. In both cases the conditions in the 

 strip above the lateral cut are exactly similar to those which exist 

 in cesophageal pieces (Child, '04^). Comparison of the history 

 of these strips with that of the oesophageal pieces will show that 

 both are affected in exactly the same way. Some degree of 

 internal pressure may be established by osmosis or secretion after 

 closure, but this gradually diminishes, the tentacles atrophy, and 

 the whole piece shrivels and finally breaks up. 



The relative change in level of the two partial discs, which 

 results in the production of a disc of typical form and in typical 

 position with respect to the body axes, is a features of special 

 interest. It is impossible to determine from the appearance after 

 regulation is completed whether the lateral supplementary disc 

 moves orally to the level of the old disc, or vice versa, or finally 

 whether both change their level, the old partial disc moving 

 aborally and the new orally until they attain approximately the 

 same level. 



There is always in a regenerating end a certain amount of 

 growth in the oral direction due undoubtedly to internal pres- 

 sure. The supplementary partial disc changes its level in the 

 oral direction in consequence of growth. This change brings it 

 nearer to the level of the old disc, which of course has not been 

 altered since its tissues are fully differentiated and not in a con- 

 dition for rapid growth. Probably this oral movement of the 

 regenerating disc is sufficient, when it is situated in the cesopha- 

 geal region, to bring it approximately to the same level as the 

 old disc. If differences in level remain after the regenerative 

 growth is completed, I think it is probable that further regulation 

 may occur in the manner discussed in connection with oblique 

 pieces (Child, '04^}, viz., through the attempt of the animal to 

 orient disc and body-axis in a definite manner. In the pieces 

 with supplementary partial discs in the cesophageal region the 

 difference of level between the supplementary disc and the old 



