ASEXUAL REPRODUCTION IN SAGARTIA 185 



The new mesentery is then similar to the old incomplete mesen- 

 teries of the latest cycle except that the regenerated ones are 

 usually thinner and have a much narrower and more lightly 

 staining plate of mesogloea (for instance, figs. 7 and 8). They 

 rapidly elongate vertically, extending downward to the base 

 and upward to and across the oral disc; but the increase of a 

 mesentery in thickness and in radial extent is slower until it 

 has become complete by crossing the oral disc to the esophagus. 

 It then rapidly extends centripetally, apparently as a result of 

 tension, since a mesentery that has recently become complete is 

 often thinner than one of the same age that has not quite reached 

 the esophagus. At about the time a mesentery becomes com- 

 plete in the extreme oral region, its longitudinal muscles become 

 clearly evident (figs. 9 and 10). Subsequently these mesen- 

 teries grow thicker and their longitudinal muscles become more 

 fully developed (fig. 13). They become attached to the esoph- 

 agus farther and farther aborally. For a long period, however, 

 at least some of the new complete mesenteries fail to reach the 

 esophagus at lower levels where the old ones are attached (figs. 

 15 and 18: c^ c^). 



The incomplete mesenteries arise in similar fashion, but they 

 develop more slowly and retain much longer the slighter degree 

 of development which distinguishes them from the incomplete 

 mesenteries of the old region. 



Number and arrangement of mesenteries in regenerated regions 



Upon examination of the mesenteric formulas of the new 

 regions of the specimens represented in tables 3 to 6, it is apparent 

 that the variations in number and arrangement of the new 

 mesenteries are determined almost wholly by the mesenteries 

 on the boundaries of the old regions. Specimens listed in 

 tables 7 and 8 give additional data upon which to base a study 

 of regeneration formulas. Some of these were selected, in col- 

 lecting, upon a basis which would interfere with their usefulness 

 in considering the relative frequency of fission planes in different 

 regions. Others were excluded from tables 3 to 6 on the ground 



THE JOURNAL OF EXPERIMEXTAL ZOOLOGY, VOL. 28, NO. 2 



