196 DONALD WALTON DAVIS 



of the first cycle unless a pair of these were included in the old 

 part. As a result, normally divided and regenerated specimens 

 occasionally lack a pair of incomplete mesenteries of the first 

 cycle between two adjacent pairs of complete mesenteries. As 

 will be shown later (p. 210), this involves the loss of an orange 

 stripe, giving rise to an uneven number of these externally ob- 

 servable features. An unexplained lack of a pair of incomplete 

 mesenteries of the first cycle between two pairs of new complete 

 mesenteries is evident in the specimen represented in figure 6. 

 Some instances have appeared which show two pairs of incom- 

 plete mesenteries of approximately equal size unseparated 

 by mesenteries of a higher grade. I have among my sections 

 perhaps half a dozen examples of this anomalous condition 

 (p. 212). I have no explanation for it. Neglecting incomplete 

 mesenteries except the single bounding one, the order of appear- 

 ance of new mesenteries on the side of the new directive plane 

 toward an old incomplete mesentery may be indicated as 

 follows: c\ d, c*, C-, c^, (1). 



Soon after c^ becomes complete orally, c^ becomes equal to it 

 in development, c^ and c^, as in the case of division in a com- 

 plete endocoel, become equal in size, but lag considerably behind 

 their mates in becoming complete. Various stages in regenera- 

 tion on this plan are represented in figure 6 (older regeneration) , 

 11 and 13 (reduced regenerations), and 15. 



I have few examples of early stages of regeneration where the 

 old bounding mesentery is a directive. Two such cases indicate 

 that the new bounding directive appears at a stage very slightly 

 in advance of the paired incomplete mesenteries, i.e., at the 

 same stage as an incomplete bounding mesentery. Examples of 

 later stages show the bounding directive fully as well developed 

 as the pair of directives in the middle of the new area. It is 

 possible that the new bounding mesentery in such cases develops 

 more rapidly in the intermediate stage about the time when it 

 reaches the region of the oral disc and esophagus. 



I have no clear cases showing early stages of development 

 after division in exocoels. One would expect the order of de- 

 velopment to be the same in such cases as when division is in 



