1 66 INDIVIDUALITY IN ORGANISMS 



the head in Planaria, in or near the median ventral 

 region of the apical end of the piece just basal to the 

 cut surface (Fig. 99). When the piece is taken from the 

 lateral margin of the plant body and does not contain 

 the median region, individuals usually arise near the 

 apical end and ventrally on the most nearly median 

 region of the piece (Fig. 100). In pieces with oblique 

 instead of transverse apical cut surfaces the position 

 of the new individual varies according as the piece 

 contains part of the midrib or not, according to the 

 obliquity of the plane of the cut, and probably also 

 according to the region of the body. Where the piece 

 does not include the midrib the new individual usually 

 arises ventrally near the most apical region of the piece, 

 the major gradient being the chief factor in determining 

 its position. Thus in Fig. 101 the new plant appears 

 near the lateral margin, undoubtedly because the meta- 

 bolic level is higher here than elsewhere. The con- 

 ditions here are apparently much like those which 

 determine the asymmetrical position of the new head in 

 Planaria in Fig. 95. In pieces which contain a part of 

 the midrib this is usually the chief factor in determining 

 the position of the new head. The piece in Fig. 102, 

 for example, is cut from one side of the body and includes 

 part of the midrib at the basal end of the oblique cut, and 

 the new bud arises here. The influence of the midrib 

 in localization in this form depends on the fact that the 

 cells in this region retain their capacity for growth and 

 division much longer than the cells of the lateral regions, 

 and so they represent a relatively high metabolic level 

 and bear much the same relation to the transverse 

 gradient that the apical growing tip does to the major 



