216 University of California Publications. [zoology 



ally happens, however, that before the division is completed, an 

 area of the foot disk near one of the free edges produced by the 

 tear, becomes secondarily attached and ceases to follow the 

 migrations of the moiety with which it is connected. A new 

 strain in the intermediate tissue results, ending in complete 

 rupture and the establishment, by regeneration, of a third polyp, 

 usually much smaller than the other two, but possessing from 

 the first a portion of the oesophagus, mouth disk and a few 

 tentacles. A fourth and rarely a fifth fragment may he formed 

 similarly before the division may he said to have given wa\ to a 

 period of repair. In the last case, the fission plane passes quite 

 irregularly with respect to the original major mouth axis. The 

 process as a whole is strikingly irregular, and appears to differ 

 from the basal fragmentation of Metridium only in so far as each 

 fragment retains a bit of the oesophagus and a few tentacles. 



(b) Regeneration succeeding fission of both foregoing types. 

 As soon as fission has been accomplished, the torn edges of the 

 body wall roll in and the wound closes, with the tentacles 

 retracted. In fission of the second type, the edges begin to roll 

 in on one side as soon as formed, without waiting for the com- 

 pletion of the division on the other side. In a day or two, each 

 new polyp now expand, and the edges of the wound may be seen 

 to have fused. Along the line of fusion a strip of new tissue 

 begins to appear, easily recognizable by its color, which is many 

 shades lighter than the rest of the body wall. This is the zone 

 of regeneration, in which new tentacles and mesenteries soon 

 make their appearance. 



The mesenteries are the first to develop, but there is no con- 

 stant relation between the appearance of mesenteries and ten- 

 tacles, the latter appearing now earlier, now later, and in no 

 absolutely fixed order. The first pair of mesenteries arises in 

 the middle of the zone, and is soon followed by two other mesen- 

 teries, one on each side of the original pair. This stage with 

 four mesenteries of approximately equal size is so frequently met 

 with that it was some time before it was discovered that they do 

 not appear simultaneously. Next, stages with six mesenteries 

 are obtained, due probably to the addition of a mesentery on 

 each side of the first four. But beyond this point we can say 



