164 



INDIVIDUALITY IN ORGANISMS 



ventral region of the apical end, this region represent- 

 ing the region of highest metabolic rate or irritability 

 resultant from the three main axial gradients. By alter- 

 ing the shape of the piece in relation to the axial gradients 

 it is possible to alter the position of this outgrowth and 

 so the position of the new head. In a piece cut very 

 obliquely {abed, Fig. 94), the head develops as in Fig. 95, 

 and the side of the head which arises from the more 



98 



Figs. 94-98. — Localization of head-formation in the reconstitution 

 of pieces of Planaria as resultant of apico-basal and transverse axial 

 gradients: Fig. 94, diagrammatic outline of part of body of Planaria, 

 indicating shapes of pieces; Fig. 95, asymmetrical position of head in 

 reconstitution of piece, abed; Fig. 96, reconstitution of piece, aehd; 

 Fig. 97, reconstitution of piece, aegi; Fig. 98, reconstitution of piece, afi. 



apical level of the piece is likely to develop somewhat 

 more rapidly than the other side. This asymmetry 

 of position and development is due largely to the fact 

 that one side of the cut surface represents a higher level 

 in the major axial gradient than the other and so reacts 

 more rapidly. When the cut surface is oblique, the 

 major gradient becomes a factor in determining the 

 position of most rapid dedifferentiation, division, and 

 new development of cells, and this determines the 



