76 Papers from the Marine Biological Laboratory at Tortugas. 



less, as the limits or periphery of the body is reached? This pressure is 

 responsible for the " gradual slowing of regeneration as the normal form 

 is approached, and it is apparent that this retardation will be the same, 

 whether it occurs near the end of an old part or as a new part approaches 

 completion." If this be true it ought also to follow that the pressure con- 

 ditions of regenerative forces are greater in the center than at the periphery 

 of the disk in Cassiopea. 



In the experiments, only the oral epithelium and thin superficial muscle- 

 layers were removed. It may be that such experiments are not conclusive, 

 since this pressure might exert itself outward from the face or cross-cut 

 area only, and not so clearly on the surface. At any rate, as will be seen, 

 the results do not lend particular strength to the idea of greater regenera- 

 tive pressure near the center. 



Six medusje were operated upon as follows: From the oral surfaces of 

 two individuals, Nos. \ and ia, two rectangular pieces of epithelium and 

 underlying muscle were removed. The removed tissues had the same width 

 in a radial direction and were equidistant from the periphery, while one piece 

 was longer than the other in the direction parallel to the circumference of 

 the disk (fig. 18). If the pressure exerts itself only in a radial direction, 

 then the two cuts should regenerate at the same rate independent of their 

 peripheral lengths, since they are equally wide. Two other meduste, Nos. 2 

 and 2.\, had two equal-sized pieces removed from their oral surfaces, one 

 piece being nearer the center than the other (fig. 19). The last two, Nos. 3 

 and 3A, had one piece running in a radial direction cut from each, as seen in 

 figure 20. 



Mayer (1906) has shown that when the epithelium is scraped away on 

 the oral surface of Cassiopea an electrical stimulus applied on one side of 

 the abrasion is unable to pass over and stimulate the tissue on the other 

 side. As soon, however, as a very delicate layer of tissue is regenerated 

 over the cut place the stimulus will be transmitted across. This affords a 

 delicate means of detecting the first trace of regeneration. 



Twenty-four hours after the above operations No. 3 was scratched on 

 its oral surface so as to divide it into a series of concentric rings (fig. 21). 

 The rings were then scratched across at a place opposite the removed radial 

 strip. The rings of tissue were thus broken at one place, so that no impulse 

 could pass from one of their halves to the other unless tissue had regen- 

 erated over the radial injury sufficiently to conduct the stimulus. It would 

 be expected that the inner ring should be the first to conduct. None of the 

 injuries had regenerated sufficiently after 24 hours. 



Two days after the operation Nos. i and ia did not transmit across 

 their injuries. No. 2 transmitted the stimulus across the inner area only, 

 although this was equal in extent to the more peripheral injury. The clear, 

 transparent regenerating epithelium could now be seen, and it was noticed 



