98 Papers from the Marine Biological Laboratory at Tortugas. 



phery of a medusa disk and will regenerate from its cut edge until the central 

 space is grown over with new tissue. Such a ring freshly cut had the 

 sense-organs removed from half of its periphery, while equal-sized pieces 

 of tissue between the sense-organs were removed from the other half, thus 

 making the degree of injury equal on the two halves. The oral epithelium 

 was then lightly scraped across between the sense-organ half and the other 

 without sense-organs. After the last operation the first half continued to 

 pulsate, while the latter came to rest, since the stimulus for pulsation seems 

 to be derived from the sense-organs and can not be transmitted across the 

 scraped epithelium (see fig. 28). This ring regenerates tissue toward the 

 center until the space is covered over. The rate of regeneration from 

 the half at rest and the half in motion is on comparisons with the controls 

 found to be the same. The results with this medusa show that activity or 

 effort is not capable of accelerating the regeneration rate, as authors have 

 held to be the case in other animals. This is the most decisive experiment 

 that I know of as a direct test of the influences of action and rest on the 

 rate of regeneration from tissues under as nearly as possible identical con- 

 ditions, being similar united portions of one individual. 



VI. Medusae having one or more of their mouth-arms removed regen- 

 erate these mouth-arms at irregular rates which are not closely associated 

 with the number of arms cut away, or, in other words, with the degree of 

 injury. Two medusae, each having three of the eight mouth-arms removed, 

 may show a greater difference between their average specific rates of re- 

 generation than would be found to exist among the average specific rates 

 of regeneration from individuals with one, two, four, or five mouth-arms 

 cut away. (See table 2.) An individual from which several mouth-arms 

 have been removed in as near as possible similar ways will exhibit as great 

 a degree of variation among the specific regeneration rates of its several 

 arms as will be found to exist among the average specific regeneration 

 rates of many individuals, each having had a different number of arms re- 

 moved. (Compare tables 3, 4, and 5 with table 2). 



Cassiopea is well fitted for experiments of this nature, since the regen- 

 erating buds of the mouth-arms grow continuously and may be measured 

 and compared at any time during the experiments. It is of advantage also 

 to have the several mouth-arms almost identical in size and form, as a com- 

 parison of the regenerative processes from the individual arms of a single 

 medusa is thus facilitated. Most of the experimental investigations per- 

 taining to the question of the relation between the degree of injury 

 and the rate of regeneration have been conducted on crustaceans. These 

 animals must molt before the regenerating bud can be observed, and since 

 the length of the molting period varies so widely among the individuals the 

 true specific rate of regeneration is difficult to estimate. The regenerating 

 bud very probably grows as much as the confining chitinous covering will 



