188 DONALD WALTON DAVIS 



teries was formed lateral to the directive pair on one or 

 both sides. 



When one of the bounding regions is occupied by a com- 

 plete and the other by an incomplete mesentery, nine, seven, 

 or five new complete mesenteries are formed. When nine 

 are regenerated (as in no. 19a, table 5 and fig. 27), the new 

 mesenteries consist of one pair of directives, a pair of non- 

 directives on either side of these, a second pair of non-directives 

 on the side toward the incomplete mesentery, and an additional 

 complete mesentery pairing with the latter on the side toward 

 the complete bounding mesentery. WTien but seven complete 

 mesenteries are formed after division in one complete and one 

 incomplete endocoel, the arrangement is the same as when nine 

 are produced except that on the side of the incomplete mesentery 

 two fewer complete non-directives are found. Sections, at two 

 different levels, of a specimen (no. 66, table 6) regenerating in 

 this way are shown in figures 12 and 13. Other specimens are 

 represented in figures 11 and 19. In the case recorded as show- 

 ing only five new mesenteries, two fewer are formed on each 

 side. 



Where a directive mesentery lies on the boundary, the new 

 bounding mesentery is a directive mating with the old one, and 

 two pairs of non-directives are formed on that side of the wholly 

 new directive pair (for instance, nos. 21a and 21b, table 4). In 

 one case (no. 10b, table 2) the new bounding mesentery adjacent 

 to the old directive was not, at the time of killing, developed to 

 a stage where it could be distinguished from an incomplete 

 bounding mesentery. 



On a side where an exocoel is involved in the division (for 

 instance, nos. 54 and 84, table 5), regeneration is precisely the 

 same as in a case where an incomplete mesentery occupies the 

 boundary, except that no new bounding mesentery is formed. 

 In none of the cases I have observed are there less than two 

 pairs of non-directives on such a side. Two peculiar and in- 

 teresting cases (nos. 18b and 22b, table 5) possibly illustrate 

 the influence of the old bounding mesenteries on the regeneration, 

 and suggests its limits. No. 22 was a specimen regenerating 



