142 INDIVIDUALITY IN ORGANISMS 



undergo fission.' These and various other methods all 

 serve merely to increase the degree of physiological 

 isolation of the basal region by decreasing the degree 

 and range of dominance. 



EXPERIMENTAL OBLITERATION AND DETERMINATION OF 

 AXIAL GRADIENTS AND DOMINANCE 



In the case of the hydroid Corymorpha (see pp. 92, 132) 

 the original gradient can readily be obliterated and the 

 establishment of new gradients determined by experi- 

 mental conditions. Reconstitution in pieces four or 

 five millimeters or more in length from the naked region 

 of the stem in sea-water under the usual laboratory con- 

 ditions is like that in most of the longer pieces of Tuhu- 

 laria stem (see Fig. 63, p. 133). A hydranth develops 

 at the apical end of the piece, and later a second smaller 

 hydranth appears at the basal end. The metaboHc 

 conditions are also similar to those in Tubularia, and 

 reconstitution can be altered and controlled in much the 

 same way in both forms. If, however, such pieces of 

 Corymorpha are placed after cutting in 2-2J per cent 

 alcohol in sea-water the cut ends heal, but hydranths do 

 not develop. In the course of a few days the pieces 

 become shorter and more rounded, decrease in size, and 

 lose the characteristic structure of the Corymorpha 

 stem. The changes in shape are indicated in Figs. 71 

 and 72. On removal to water after several days in 

 alcohol a new hydranth begins to develop on the upper 

 side of the piece (Fig. 73), then a stem arises below it, and 



^ Child, "Physiological Isolation of Parts and Fission in Planaria," 

 Archiv fiir Entwickehingsmechanik, XXX (Festband fiir Roux), II, Teil, 

 19 10. 



