DOMINANCE IN RECONSTITUTION 335 



ated Corymorpha cells the apical region shows a similar independence of 

 other parts (Fig. 117, p. 347). It is possible, of course, that these partial 

 apical forms fall short of complete development in some, as yet unrecog- 

 nized, structural or functional characteristic, but no such lack is evident. 

 The hypostome reconstituted from a short piece is capable of reacting to, 

 and taking in, food, although the food immediately passes out at the 

 proximal end. 



Figure 113, J-M, shows interesting bipolar forms reconstituting from 

 the stalk of the sessile scyphozoan Halidystus (see Fig. 23, p. 50). The 

 stalk differs rather widely in structure from the umbrella, but tentacle 

 groups and marginal organs develop directly from the healed surface 

 of section, and with simultaneous distal and proximal section frequency 

 of bipolar forms is high (Watanabe, 1937). Further changes involved in 

 reorganization of the stalk into an umbrella are much less rapid; and 

 sometimes dominance is too weak to bring them about, and the piece 

 remains essentially a stalk with tentacle groups at one or both ends 

 (Child). 



It has already been pointed out that a hydranth or head, reconstituting 

 at the distal or anterior end of a piece from any level except that immedi- 

 ately adjoining the original hydranth or head, is "out of place," as truly 

 "heteromorphic" as a hydranth or head developing at the proximal or 

 posterior end of a piece. At either end they are hydranths or heads of new 

 individuals and determine reorganization of pattern over a greater or less 

 distance. When the hydranth or head develops at the distal or anterior 

 end of the piece, the new polarity is in the same direction as the old and 

 determines development of parts apical or anterior to the level of origin 

 of the piece; but when it develops from the proximal or posterior end, it 

 determines a polarity opposite in direction to that originally present. The 

 evidence supports the conclusion that hydranth or head is not determined 

 by other parts of the piece, for both can develop when other parts are not 

 present or in a relation to other parts quite different from the normal. 

 On a postoral planarian piece, for example, a head begins to develop be- 

 fore pharyngeal, oral, and prepharyngeal levels. If reconstitution were 

 determined anteriorly from more posterior levels, we should expect that 

 oral, pharyngeal, and prepharyngeal regions would develop successively, 

 and finally the head; but this is never the case. The same relations appear 

 in annelid reconstitution. Whatever the level of section, in species capa- 

 ble of head regeneration a head regenerates first, and in most species 

 only a certain number of segments characteristic for the species regener- 



