Factors of Form Regulation in Harenactis Attenuatn 497 



The mesenteries of the second cycle are smaller than those of the 

 first, their retractor muscles are less developed and they bear 

 gonads only occasionally. Both cycles, however, extend the v^^hole 

 length of the column, so that twenty-four mesenteries are present 

 in any cross-section. Thus no new mesenteries are formed in 

 restitution from a distal cut surface at any level and the number 

 of tentacles never exceeds and is very rarely less than twenty-four. 



Pieces from the proximal fifth or fourth of the body, however, 

 when they produce tentacles at all produce a smaller number than 

 twenty-four. Usually such pieces simply close after a consider- 

 able time but produce no tentacles. In only four cases of this 

 kind has the formation of tentacles been observed. 



One of these cases, a piece consisting as nearly as could be deter- 

 mined, of that portion of the body proximal to the line e in Fig. 2, 

 gave rise at first to ten tentacles and very soon after to two more, 

 twelve in all (Fig. 23). The ten tentacles were first visible four- 

 teen days after section and the two additional appeared within 

 the following week. Of the twelve tentacles as shown in Fig. 23 

 eleven are about equal in length and one is much shorter. A 

 month later the piece possessed fourteen tentacles, thirteen long 

 and one short. Unfortunately the position and sequence of new 

 tentacles was not determined. Though the piece w^as kept for 

 two months more no more tentacles were produced. 



A second case in which the piece included approximately the 

 region proximal to the line / in Fig. 2, gave rise after twenty davs 

 to twelve tentacles which attained a length of 5-6 mm. No addi- 

 tional tentacles were produced by this piece and the tentacles 

 present underwent gradual reduction during two months following, 

 after which the piece began to degenerate. 



In two other cases in whi h the level of section was somewhere 

 in the region of the line d. Fig. 2, the tentacles appeared after six- 

 teen days. Their number was not determined at first, but ten 

 days later it was found to be twenty-one in each case. A diagram 

 of the disc of one of these cases is shown in Fig. 24: fifteen long and 

 six short tentacles are present, and it is clear that the short ten- 

 tacles belong to both cycles and are not definitely related as re- 

 gards position to the siphonoglyphe. In the other case similar 



