I02 Papers from the Marine Biological Laboratory at Tortugas. 



trols, some of which, as in Zeleny's experiments, consisted of an entire pul- 

 sating ring and others of the entire ring at rest. 



The level at which the margin is removed determines the rate of regen- 

 eration from the cut surface, and this possibility is not mentioned in 

 Zeleny's account. It is difficult to remove a marginal strip of exactly the 

 same proportions from several medusre, even when careful measurements 

 are made. Slight differences in the depths of the cuts would easily account 

 for the results, but since the rates of regeneration from both the center and 

 outer margin are considered by Zeleny this is in a manner checked. In 

 my " ring preparations " described above, the level of the cut is controlled 

 as far as possible, since the disk center also forms the center of the circular 

 mass of tissue that is removed. Thus the cut surface and the disk peri- 

 phery are concentric circles; therefore, all places on the cut surface are at 

 a common level. My observations are aided by the fact that the pulsa- 

 tions were continuous throughout the experiment, while Zeleny's rhyth- 

 mical pulsations always stopped before the end of his experiment. 



