ASEXUAL REPRODUCTION IN SAGARTIA 227 



No obvious change occurs in the old part in consequence of 

 division, except that possibly mesenteries injured in the process 

 of fission are eliminated by absorption (pp. 168, 171, 172, 200). 



Regeneration processes begin with the rolling in and fusion of 

 the torn edges of the body wall (pp. 167, 182). In the region of 

 fusion new structures are gradually differentiated, eventually 

 constituting a large proportion of the bulk of the individual — 

 often far the greater part (pp. 167, 182). 



The torn edges of the esophagus also grow together and a new 

 siphonoglyph invariably becomes differentiated in the region of 

 fusion of these edges. As regeneration proceeds, the new siphono- 

 glyph occupies the middle of the new region, thereby marking 

 this as a new directive plane (p. 183). In those instances where a 

 siphonoglyph is cut by the fission plane (p. 183), a siphonoglyph 

 occupies the corresponding boundary between old and new parts 

 of the regenerated animal and a wholly regenerated siphonoglyph 

 is formed in addition. Origin of specimens with different num- 

 bers of siphonoglyphs from any of the common types is com- 

 pletely explained by the manner of division and regeneration 

 (p. 183). 



Four new mesenteries, constituting a very characteristic group, 

 become established in the middle of the new region of the 

 column and grow across the oral disc to the esophagus (p. 193). 

 Longitudinal muscle swellings appear on each of the first four 

 mesenteries on the side away from the directive plane (p. 194). 

 Additional mesenteries follow in a bilaterally paired manner 

 lateral to the first set of four (pp. 193, 198, 207). Among these 

 later mesenteries certain members mate with the two outer ones 

 of the set of four, forming unilateral pairs. The two inner 

 members of this set of four constitute a pair of directive mesen- 

 teries. Other of the later mesenteries become paired with the 

 old bounding mesenteries or with each other, so that eventually 

 all of the mesenteries, with the exception of the directives, are 

 present in the unilateral pairs characteristic of the Hexactinians 

 (pp. 193, 207). 



Variations in the number of mesenteries formed depend almost 

 wholly upon the character of the old bounding mesenteries. On 



