204 DONALD WALTON DAVIS 



almost without exception, according to the biradial plan; but 

 pieces containing material from the base alone frequently de- 

 velop on a bilateral plan. In S. viduata the same influence of 

 the character of the material in the regenerating piece is found 

 with a greater tendency, whatever the nature of the piece, 

 toward a bilateral plan of development. It may be that the 

 rounding up of the more homogeneous material of basal pieces 

 results in conditions similar to those influencing the development 

 of mesenteries in ontogeny. 



Siphonoglyphs and directive mesenteries 



We cannot go far in considering the relation between these 

 different types of mesenteric development without being con- 

 fronted with the more general question of the determination of 

 form in ontogeny and in regeneration. Carlgren's discussion of 

 the relation of Types I and II suggests one of these, i.e., the 

 governing influence of the siphonoglyph. He puts forward ('09, 

 p. 43) the idea that, upon the formation of the siphonoglyph 

 during regeneration, this structure immediately determines 

 that the pair of mesenteries latest formed in the same plane shall 

 be directives, and that no other mesenteries shall be formed 

 in this plane. According to this view, the difference between 

 Types I and II is due to the earlier stage at which the new 

 siphonoglyph is established in II. The latest bilateral pair of 

 mesenteries formed at the time of the appearance of the siphono- 

 glyph becomes the pair of directive mesenteries, and no new 

 bilateral pairs arise except those mating with mesenteries already 

 present to form radially placed non-directiye pairs. 



My observations lend some support to this hypothesis in so 

 far as it involves the determination of the directives by the 

 siphonoglyph; but I believe that in S. luciae delay in formation 

 of a siphonoglyph does not lead to the production of more than 

 two bilateral pairs of mesenteries adjacent to the potential 

 directive plane. As I have already indicated, the siphonoglyph 

 appears shortly after closure of the wound in the region of the 

 mouth. At about this time some of the new mesenteries reach 



