364 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



the posterior zooid; consequently, there is Httle or no gradient difference 

 at the two ends of a short piece from this region, and neither is dominant. 

 A planarian head sectioned transversely at the level of the cephaHc ganglia 

 sometimes becomes bipolar by development of a second head opposed in 

 orientation from the cut surface (Fig. 122, F). 



Bipolarity in short planarian pieces from other levels than the fission 

 zone can be greatly increased by differential inhibition with various 

 agents (Rustia, 1925). According to the gradient concept, the two cut 

 ends of such pieces differ but little because only a small fraction of the 



Fig. 122, A-T. — Bipolar and apolar forms from short planarian pieces. A-F, Dugesia; 

 G, Cercyra papulosa, a marine triclad (after Lus, 1926); H, I, apolar forms. 



polar gradient is included between them. Differential inhibition decreases 

 this difference still further, so that after return to water the two ends are 

 so nearly alike that neither can dominate the other; consequently, the 

 activation at each end is independent of that at the other, and the new 

 tissue develops into a head and determines a new polarity as far as length 

 of piece, scale of organization, and presence of the other polarity permit. 



Under the usual conditions and under most experimental conditions 

 thus far employed, bipolar heads do not develop on pieces 1/4 or more 

 of the body length of large, mature planarians; but in somewhat hyper- 

 tonic modified Ringer solutions bipolar heads frequently appear on such 

 pieces (Child, unpublished). Since loss of motor co-ordination and more 

 or less complete paralysis occur in these solutions but regeneration is not 

 much inhibited, the decrease in dominance results in physiological isola- 

 tion of the posterior cut end to a degree permitting the activated cells 

 there to develop a head instead of a posterior end. 



Extremely short planarian pieces often appear to be completely apolar. 



