198 DONALD WALTON DAVIS 



reduced regeneration, and may point significantly to c- and c^ as 

 the mesenteries omitted in the reduced type of regeneration. 

 Comparing formulas no. la and no. 2a as given above, we find 

 that the third mesenteries to appear are different. Under these 

 circumstances it is impossible to decide which mesenteries of the 

 two formulas are truly homologous. 



It will be noticed that, as described above, the members of a 

 regenerating pair of complete mesenteries, except directives, do 

 not appear simultaneously. Although attaining eventually to a 

 condition of approximately radial symmetry, complete mesen- 

 teries arise in regeneration in bilateral fashion, a member on one 

 side of the directive plane corresponding in degree of develop- 

 ment with one in similar position on the opposite side of the 

 directive plane. There is no reason to suppose that the same is 

 not true of the development of mesenteries in the metamorphosis 

 from the larval state. The complete mesenteries of S. luciae, 

 therefore, are all to be regarded as primary mesenteries, belong- 

 ing to the first cycle of mesenteries, which is very generally 

 found arising in a bilateral manner in the ontogenetic develop- 

 ment of Hexactinians (cf. p. 207). Perfect bilateral symmetry of 

 a regenerating region is often prevented through the influence of 

 unlike old bounding mesenteries, or by unknown factors which 

 cause the suppression, on one side of the directive plane, of 

 mesenteries present on the other. 



Results of repeated fission and regeneration 



It has been shown that the products of fission in a group of 

 specimens of S. luciae vary greatly in numbers of siphonoglyphs 

 and of complete mesenteries. The former were found in speci- 

 mens recorded in tables 3 to 7 up to three, the latter up to 

 thirteen. It has been shown, further, that there is added in 

 regeneration a set of structures including a siphonoglyph and a 

 pair of directive mesenteries, together with other mesenteries 

 including mates to the old bounding mesenteries. The com- 

 plete mesenteries on either side of the new directives may vary 

 from one to five, making possible a total addition in regeneration 



