Rate of Regeneration in Cassiopca xamachana. 75 



from ; this is also true for the fish's fin, where the deeper cut has a thicker 

 base. Two measurements of the regenerating tissue were made, since the 

 thick base was not exactly a perpendicular surface. The one was from the 

 edge of the old tissue on the aboral surface to the edge of the new tissue, 

 which measured 5 mm. over the deep-cut surface ; the other measurement 

 was from the oral border of the old tissue to the margin of the regenerated 

 tissue, 3.5 mm. wide. The rate of regeneration was fastest at the corners 

 in these cuts, being 7 mm. wide at this place in the deep cut. The shallow 

 cut showed 1.5 mm. of new tissue from its middle and 5 mm. from its cor- 

 ners (fig. 17). After 9 days the deep cut had regenerated tissue 7 mm. 

 wide from its middle, while the shallow cut showed only 3.5 mm. of tissue. 

 Both of the cuts were at this time 13 mm. in width peripherally. When 18 

 days old the cuts were 10 mm. across between the vertical edges of the old 

 tissue, the deep cut had regenerated new tissue 7 mm. wide and the shal- 

 low 3.5 mm., or half as much. Here again regeneration proceeds in one and 

 the same individual at a faster rate from the cut surface at the level nearer 

 the disk-center than from a similar more distal cut. 



I may now cite an experiment which was made to test whether medusae 

 would regenerate their sense-organs faster when consecutive ones were re- 

 moved or when alternate ones were cut away. The experiment threw no 

 light on this question, but the result was curious and for this reason may be 

 mentioned. Two healthy medusas, one with 16 and the other with 17 mar- 

 ginal sense-organs, were treated as follows : Four adjacent sense-organs were 

 removed from one part of the disk and three alternate ones from the region 

 opposite these. After 23 days no definite trace of regenerating sense-organs 

 could be detected, so all of the remaining old sense-organs were cut away 

 to ascertain whether the new ones were sufficiently regenerated to maintain 

 the pulsation of the disks. The disks became perfectly still after the last 

 one of the original sense-organs was cut oflf, and only after a period of 6 

 days was one individual slowly pulsating. This is peculiar, as when the 

 entire peripheral border with all sense-organs is removed the newly regen- 

 erated tissue causes the disk to pulsate usually after two or three days. 

 Further, a number of medusas with regenerated margins had produced 

 sense-organs from their new tissue, while the two above had not regenerated 

 them from their old bases. 



REGENERATION AFTER THE REMOVAL OF PIECES OF ORAL EPITHE- 

 LIUM OF DIFFERENT SIZES AND AT DIFFERENT DISTANCES FROM 

 THE DISK-CENTER-THE QUESTION OF "REGENERATIVE PRES- 

 SURE." 



These experiments were carried out with the hope of testing whether 

 or not "regenerative pressure," in the sense Morgan (1906) used the term, 

 actually exists and exerts itself from the center radially to the periphery. 

 In other words, is this force felt more towards the center and gradually 



