182 DONALD WALTON DAVIS 



regenerated individuals. Regenerative processes may begin with 

 pieces varying in number of siphonoglyphs from none to three, 

 and in number of complete mesenteries from zero to fifteen. 



COURSE OF REGENERATION 



The course of regeneration as seen in the living animal has 

 been described above (p. 167 ff). The account which follows is 

 derived from the study of sections. It should be noted that the 

 white lines and orange stripes fade out immediately upon killing 

 by any of the methods I have used. The external distinctions 

 between new and old tissue are also lost in the process of fixation 

 by some methods, but in formalin the newer area remains dis- 

 tinguishable by its lighter color, at least in those cases where 

 the color differences in the living specimens are great. There 

 is, nevertheless, little uncertainty as to the relation of external 

 and internal features. After examining a series of stages in 

 regeneration the boundaries between new and old material can 

 be quite as accurately determined in sections as in living indi- 

 viduals, and the external markings and internal structures can 

 in almost all cases be satisfactorily correlated. 



In sections of specimens killed a few days after division the 

 column wall in the region of fusion appears much like the old 

 column wall. In some specimens this region is distinguishable 

 on account of its being slightly thinner and having the outer 

 border of the ectoderm less scalloped. This condition occa- 

 sionally extends slightly into a part of the column which, as 

 indicated by the mesenteries, is old. In the greater number of 

 cases, practically no evidence of the region of fusion is found in 

 the column wall a few days after division. In the esophagus 

 evidence of the division persists much longer, especially at its 

 aboral extremity. Closure of the esophagus orally occurs within 

 four days. At this time its torn edges are contracted, the 

 esophagus being much shorter on that side than on radii distant 

 from the plane of division. Its full length on the regenerating side 

 seems to be reached only as the new complete mesenteries become 



