gS INDIVIDUALITY IN ORGANISMS 



of the animal, the length of the piece, and the level of 

 the stem from which it is taken. The shorter the piece 

 from a given level of the stem the more completely is its 

 development limited to apical parts, as Fig. 47 shows. 

 The shortest pieces give rise to nothing but the apical 

 ends of the hydranths, with mouths and the apical row 

 of tentacles. In no case do such pieces produce basal 

 parts of the hydranth without apical parts. Where 

 anything is missing it is always the more basal region, 

 either stem or more or less of the basal hydranth region. 

 The results in such pieces constitute, I believe, a demon- 

 stration that the apical region of the individual arises 

 first and other regions are determined later, as far as the 

 length of the piece permits. 



The development of hydranths or apical portions of 

 hydranths may occur at one or both ends of such short 

 pieces as indicated in Fig. 47. This difference arises 

 according as the original metabohc gradient in the stem 

 is more or less marked. In such short pieces of the 

 stem the difference in metabolic rate at the two ends of 

 the piece is but sHght in any case. If, however, the rate 

 at the apical end of the piece is enough higher than that 

 at the basal end, development at the apical end pro- 

 ceeds more rapidly than at the basal end, the apical 

 end is dominant, and the piece produces a single hydranth 

 or part. But if the gradient is very slight in the piece 

 the two ends react at the same rate, and since the 

 presence of the wound at each end brings about an 

 increase in metabolic rate at each end, equal or nearly 

 equal gradients in opposite directions arise and hydranths 

 or apical parts arise at both ends with their axes opposed. 

 Often, even in such cases, the original gradient appears 



