CERTAIN GENERAL CHARACTERISTICS 49 



the "esophageal" region proximal to the branchial chamber develop into 

 bipolar forms with a branchial chamber at each end, but distal develop- 

 ment is more rapid than proximal (Brien, 1930; Pasquini, 1933). 



THE QUESTION OF REGENERATION IN RELATION TO DEGREE OF INJURY 



Increase in rate of regeneration with increase in size or number of parts 

 removed has been reported for certain forms and has raised the question 

 whether increase in degree of injury increases the rate.'^ In this connec- 

 tion it is of interest to inquire whether the differences in rate of recon- 

 stitution at different levels of the longitudinal axis noted earlier in this 

 chapter are similarly related to degree of injury. Rates of hydranth re- 

 constitution in hydroids and of apical reconstitution in other coelenterates 

 and rate of head regeneration in planarians and annelids decrease from 

 anterior to posterior levels, that is, with increase in size of the part re- 

 moved anteriorly, obviously an increase in degree of injury. Evidently 

 this increase is not concerned in these cases in determining rate of regen- 

 eration, for the rate is highest when the injury is least. Posterior regen- 

 eration in planarians and annelids is usually absent, incomplete, or slow at 

 levels close to the head, that is, when degree of injury is greatest; at levels 

 a little farther posterior, with a lesser degree of injury the rate is highest 

 and decreases from these levels posteriorly. This decrease might perhaps 

 be regarded as related to decrease in degree of injury if head regeneration 

 did not decrease in rate in the same direction with increasing degree of in- 

 jury anteriorly. If rate at either end is affected by degree of injury at both 

 ends, little difference in rate in pieces of equal length from different levels 

 should occur, for degree of injury is much the same in such pieces; if the 

 part removed anteriorly is small, the posterior part removed is large, and 

 vice versa. In short, these differences in rate at different levels of the 

 longitudinal axis appear to be related to inherent graded differences in 

 physiological pattern at different levels rather than to degree of injury. 

 Moreover, the scale of organization of the oral hydranth in Tuhularia and 

 Corymorpha is largest when the degree of injury at the oral end of the piece 

 is least, and that of the aboral hydranth is largest when degree of aboral 

 injury is greatest. It appears probable that in cases of apparent relation 

 between rate of regeneration and degree of injury, for example, the in- 

 crease in rate in the starfish with removal of an increasing number of 

 arms, the increasing disturbance of function, is a stimulus to regenerative 

 growth. 



'^Zeleny, 10050, b, 1909; Ellis, 1907, 1909. 



