RATE OF REGENERATION IN CASSIOPEA 7 



of specimens were used two difficulties were met in making the 

 measurements. Frequently the disk became folded backward at 

 the point where the subumbrella ectoderm was removed some- 

 times even bringing the exumbrella surfaces in contact. While 

 this seemed in no way to interfere with regeneration it frequently 

 made accurate measurements impossible unless the specimen 

 was first narcotized, as any attempt to unfold the active disk 

 usually resulted in tearing the delicate regenerating tissue. As 

 this procedure involved the expenditure of so much time all 

 badly folded specimens were discarded. If the folding took 

 place some days after the start of any series of experiments the 

 specimen w^as discarded and the figures for the earher stages re- 

 tained in the record. 



The other most common source of difficulty in making the 

 measurements arose on account of the tendency of the edge of 

 the regenerating tissue to fuse with the edge of the old cut sur- 

 face or with a more proximal part of the sheet of regenerating 

 tissue. WTienever the edge of the thin sheet of new tissue be- 

 came folded back sufficiently to touch any of the more proximal 

 tissues fusion took place so that a tube would be formed from the 

 new tissue. When the folding involved only a small area sepa- 

 ration could be easily accomplished, but if a considerable por- 

 tion of the regenerating sheet was involved the specimen was 

 rendered useless for further study. 



The results of two typical experiments are shown in table 1. 

 The measurements are in millimeters. The upper figure for each 

 date shows the wddth of the sheet of tissue regenerated from the 

 active half disk, the lower figure the width of that regenerated 

 from the passive half disk. When the sheet of new tissue had 

 entirely closed over the cavity in the center of the disk the point 

 of closure remained recognizable for at least a day so that the 

 measurements could readily be made for those disks that had be- 

 come closed since the time of the last measurements. By the 

 end of twenty-four hours after the new sheet of tissue was com- 

 pleted the point of closure would be shifted until it came to lie 

 in the center of the disk. 



