Regeneration 



173 



tion through these elements is responsible for this 

 polarity, though this may be possible. The writer 

 found, however, that one condition is necessary for the 

 growth and regeneration of tentacles which also plays a 

 r61e in the corresponding phenomena in plants, namely 

 turgidity. The tentacles of Cerianthus are hollow 

 cylinders closed at the tip, and by 

 liquid being pressed into them they 

 can be stretched and appear turgid. If, 

 however, an incision is made in the 

 body, the tentacles above the incision 

 can no longer be stretched out. In one 

 experiment the oral disk of a Cerian- 

 thus was cut off; very soon new tenta- 

 cles began to grow at the top, and after 

 having reached a certain size, an incision 

 was made in the animal. The tentacles 

 above the incision collapsed in conse- 

 quence and ceased to grow, while growth 

 of the others continued. On the lower 

 edge of the incision new tentacles began to grow. 



It seems also possible that Morgan's well-known ex- 

 periment on regeneration inPlanaria can be explained by 

 a flow of substances. He 1 found that if a piece a cd b 

 be cut out of a fresh-water Planarian at right angles 

 to the longitudinal axis (Fig. 27), at the front end a new 

 normal head, at the back end a new tail, will be regen- 



1 Morgan, T. H., Regeneration, New York, 1901. 



Fig. 27 



