170 DONALD WALTON DAVIS 



in all stages of regeneration show that the exact position of the 

 boundary between the old material and that laid down follow- 

 ing division can be distinguished, even after a long period of 

 regeneration, by the relative size and development of the mesen- 

 teries of corresponding cycles (figs. 11 to 16). In many instances 

 where the precise position of the boundary cannot be determined, 

 the relative condition of the mesenteries, the degree of develop- 

 ment of the mesenteric filaments, and the presence of certain 

 types of arrangement of the developing mesenteries peculiar to 

 zones of regeneration indicate with certainty that division has 

 occurred. Thus in figure 15 the mesenteries labeled c- and 

 c^ lag behind the other members of their respective pairs in 

 becoming attached to the esophagus. A similar condition is 

 represented in figure 18 and constitutes the sole evidence that 

 regeneration is in progress. In the latter instance as well as in 

 one half of the specimen illustrated in figures 15 and 16 the 

 exact position of the plane of division cannot be determined. In 

 figures 19 to 24 the condition of the incomplete mesenteries gives 

 evidence that different regions are of different ages without 

 indicating the precise position of the boundary between newer 

 and older parts. It is obvious that only such specimens as 

 reveal the exact boundary between new and old structures 

 should be included in table 6, but not even all of these can fairly 

 be counted. As indicated by the figures just cited, evidence of 

 division ' commonly persists longest in the incomplete mesen- 

 teries. Hence at a late stage of regeneration, following a 

 division which does not involve a pair of incomplete mesenteries, 

 there may be doubt as to whether one or both of the members 

 of a pair of complete mesenteries lying near the boundary belong 

 to the old region. If only one is old, the division was in a com- 

 plete endocoel; if neither is old, the division must have been in 

 an exocoel. See, for instance, the older regenerating region of 

 the specimen shown in figures 15 and 16, where the mesenteries 

 marked c are probably (but not certainly) old. Such a case 

 cannot be counted, of course. But specimens in a correspond- 

 ingly late stage of regeneration after a division between incom- 

 plete mesenteries, although they do show the precise position of 



