PHYSIOLOGICAL DOMINANCE 99 



in the smaller size or more incomplete condition of the 

 structure formed at the basal end of the piece. In Fig. 

 47 one case near the bottom of the figure is shown in 

 which one end is a hydranth with both sets of tentacles, 

 the other a partial hydranth w^ith only the apical set and 

 the reproductive organs.' 



In Planaria the development of short pieces is 

 essentially similar. Short pieces from various levels of 

 the body may undergo complete transformation into 

 single or double heads without the formation of other 



4>> 



Fig. 48. — Reconstitution of single and biaxial apical structures from 

 short pieces of Planaria, to illustrate independence of apical region. 



parts of the body or with more or less of the anterior body- 

 region (Fig. 48). When a single head arises, it is at the 

 anterior end of the piece. The conditions determining 

 the development of these heads are the same as those in 

 Tuhularia. As in Tubularia also, the original gradient 

 may appear to some extent in the more rapid and more 

 complete development, larger size, and dominance in 

 motor activity of the head at the original anterior end 

 of the piece, as in the case at the right of Fig. 48. 



^ For more extended consideration see Child, "Analysis of Form- 

 Regulation in Tuhularia. V, Regulation in Short Pieces," Archiv fiir 

 Entwickelungsmechanik, XXIV, 1907; "Die physiologische Isolation 

 von Teilen des Organismus," Vortrage und Aufsiitze iihcr Eiitwickduugs- 

 mechanik, H, XI, 191 1, 101-19. Further references are given in these 

 papers. 



