ASEXUAL REPRODUCTION IN SAGARTIA 195 



Whether the old bounding mesentery is complete or incom- 

 plete, soon after the appearance of the mesenteries destined to 

 become complete and about the time these reach the esophagus, 

 pairs of the first order of incomplete mesenteries appear in the 

 exocoels between d and c^ and between c^ and c\ This is shown 

 in an early stage on the right sides of figure 13 and of figure 11, 

 and in the older regenerating regions of figures 16 and 6. In some 

 cases these pairs appear simultaneously, but commonly the pairs 

 nearest the directives are slightly in advance of the pairs nearer 

 the boundary. Considerably later, pairs of a second cycle of 

 incomplete mesenteries appear in their characteristic positions, 

 alternating with the pairs of both the complete and the incom- 

 plete mesenteries of the first order. See, for instance, figures 17, 

 27, and 28. In cases where the old bounding mesentery is an 

 incomplete mesentery of the first grade, the new one (/) mating 

 with it appears about the same time as the first cycle of 

 incomplete mesenteries in the regenerating part, but in nearly all 

 cases as the first of the mesenteries of this order. It remains the 

 largest mesentery of its cycle for a considerable period. This 

 mesentery is shown in figures 13, 11, 16, and in the older re- 

 generating region of figure 6. When, on the other hand, the 

 old incomplete bounding mesentery is of the second order, the 

 new incomplete mesentery {II) pairing with it appears much 

 later, at about the same time as the incomplete mesenteries of 

 the second grade in other parts of the new region, usually as the 

 first representative of this cycle. Two such new incomplete 

 mesenteries may be seen in figure' 17. An apparent exception 

 is noted in connection with no. 80, table 7. It thus happens 

 that one old and one regenerated mesentery, constituting a pair, 

 may be definitely assigned to a certain cycle. Furthermore, the 

 bounding mesentery is the only one of this cycle to be produced 

 between the boundary and the nearest pair of new complete 

 mesenteries, no incomplete mesenteries of higher grade being 

 formed in this space. Consequently, after regeneration follow- 

 ing division in an incomplete endocoel of the second order, the 

 space including the boundary and lying between two adjacent 

 pairs of complete mesenteries will lack incomplete mesenteries 



