DOMINANCE IN RECONSTITUTION 



337 



H 



H 



H 



the reconstituted partial disks and the complete disk at the distal end 

 after complete transverse section are evidently directly determined by 

 the original radial pattern. Under other conditions, 

 however, reconstitution of a radial or of a bilateral 

 pattern is possible in this animal quite independent- 

 ly of the original radial pattern (pp. 371-74). In 

 planarians the anterior tip of a head regenerating 

 from a complete transverse section is directed away 

 from the piece, and its dorsiventrality coincides with 

 that of the piece ; but a complete head may regener- 

 ate from a surface of section half, or less than half, 

 the width of the body or even from a longitudinal 

 cut surface (Beyer and Child, 1930). A complete 

 head may also regenerate from a partial transverse 

 section in various annelids (Morgan, 19026; von 

 Haffner, 193 1, etc.). 



Von Haffner's experiments on Lumbriculus are of 

 special interest. Following removal of lateral pieces 

 from different body-levels with injury of axial or- 

 gans, heads developed from the more anterior levels, 

 both heads and posterior ends from middle levels, 

 the posterior ends anterior to the heads, and only 

 posterior ends from posterior levels (Fig. 114). In 

 the middle region, where both heads and posterior 

 ends develop, the heads decrease in size and devel- 

 opment and posterior ends increase in size from more 

 anterior levels of section posteriorly (Fig. 114, A', 

 F, Z). In this region posterior ends are anterior to 

 heads because heads develop from the cut surfaces 

 facing anteriorly, posterior ends from those facing 

 posteriorly. As von Haffner suggests, these relations 

 are doubtless due to the polarity of the original 

 animal, but it is perhaps possible to go a step farther 

 and suggest that differences in degree and proba- 

 bly in rate of activation and in dominance at differ- 

 ent body-levels are concerned. At the more anterior 

 levels the regenerating head dominates the new tissue completely, and 

 only an anterior end develops. At the middle levels dominance of the re- 

 generating head is less effective, and the dominance of the original ani- 



FiG. 114. — Lumbricu- 

 lus variegalus, diagram- 

 matic, indicating char- 

 acter of regeneration in 

 relation to body-level 

 after removal of small 

 lateral pieces with in- 

 jury of axial organs; H, 

 head; P, posterior end 

 (after von Haffner, 1 93 1 ) . 



