336 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ates posterior to the head, even though more were removed (e.g., Hy- 

 man, 1916a). In general it appears that a necessary condition for hy- 

 dranth or head formation on an isolated piece is an activation of the 

 cells concerned sufficiently intense to bring about a high degree of 

 physiological isolation from other parts. When they become independ- 

 ent of other parts, they reconstitute a new dominant region and so 

 begin development of a new individual. Any level of the Tuhularia or 

 Corymorpha stem, any postcephalic level of the planarian, and all, or the 

 more anterior, postcephalic levels of many nemerteans and annelids will 

 develop as hydranth or head if not dominated by some other part. In 

 other words, so far as other parts of the body are concerned, the hydranth, 

 or even its apical portion, and the head, or probably the nervous tissue 

 of the head, appear to be self-determining and self-differentiating sys- 

 tems. They may be regarded as expressions of the primary or funda- 

 mental action system of the species, at least so far as reconstitution is 

 concerned. In the reconstitution of various minor or subordinate and spe- 

 cialized axes, such as appendages of various sorts, relations as regards 

 the longitudinal or polar axis of the part are apparently essentially similar, 

 at least in earlier stages. The distal tip of the starfish arm regenerates 

 "out of place" as regards relations to other parts and basipetal deter- 

 mination of the axis follows.^ Regeneration of a whole distal part of an 

 amphibian limb may take place from a half-transverse section (p. 370), 

 and a distal part may even regenerate from a proximal surface of section 

 of a limb implanted with longitudinal axis reversed (p. 390). 



Position of a hydranth or head at the distal or anterior end of a piece 

 may be determined by the original polarity of the piece, though it is not 

 necessarily so determined; but the polarity of the developing hydranth 

 or head is evidently independent of the original polarity, even when in 

 the same direction, for it does not "fit" into that polarity but makes it 

 over more or less completely, according to body-level of the piece. The 

 original symmetry or asymmetry of the piece may play a part in deter- 

 mining symmetry or asymmetry of the reconstituting dominant region 

 (pp. 387-89) but does not necessarily do so, for radial symmetry of hy- 

 dranth and dorsiventrality of head may develop in various positions with 

 respect to the original symmetry. 



The actinian Harenadis affords interesting examples of development of 

 symmetry in reconstitution. Partial disks develop after partial transverse 

 section of body wall and esophagus (pp. 387-88) ; here radial pattern of 



=! King, 1898, 1900; DawydofiF, 1901; Morgulis, 1912; Schapiro, 1914. 



