190 DONALD WALTON DAVIS 



From these statements it appears that the chief variations in 

 number and arrangement of mesenteries on each side of a re- 

 generating area are wholly dependent upon the old bounding 

 mesentery on that side, except for two complete non-directives, 

 which may or may not be present whatever the character of the 

 bounding mesentery. 



Five specimens showing exceptional features in their regenera- 

 tion are represented in table 8. Two of these specimens show 

 an abnormally large number of new mesenteries, — one (no. 88) 

 a second pair of directives, the other (no. 89) a second pair of 

 directives and two additional pairs of non-directives. A third 

 (no. 87) shows a new region which is normal except for the weak 

 stage of development of the muscles in the pair of mesenteries 

 in the position ordinarily occupied by the directives. These 

 mesenteries extend to the esophagus, but their longitudinal 

 muscles are so weakly developed that the directive or non-direc- 

 tive character of the pair cannot be determined. The fourth 

 (no. 86) shows this pair of mesenteries extremely reduced. They 

 do not reach the esophagus and show no evidence of longitudinal 

 muscles. The fifth (no. 85) shows a reduction from the usual 

 number of mesenteries, there being only one new bounding 

 directive and two pairs of non-directives. An old regeneration 

 zone in no. 40 (table 5) shows an excess of two mesenteries over 

 the usual number. 



Neglecting the few exceptions (six in all) just referred to, we 

 may say that in regeneration a new pair of directives is formed, 

 approximately in the center of the new zone, and that on each 

 side of this pair of mesenteries are formed one or two pairs of 

 complete mesenteries plus an odd mesentery if required to mate 

 with a bounding mesentery on the edge of the old part. Treating 

 separately each lateral half of a new zone, the tendency in full 

 regeneration is to produce on each side of the middle plane a 

 directive and three non-directive mesenteries; if a complete old 

 bounding mesentery is not present, additional mesenteries are 

 formed to mate with the odd new non-directive and with any 

 old incomplete bounding mesentery. 



