210 SENESCENCE AND REJUVENESCENCE 



that there is much evidence in favor of this view and none which 

 seriously conflicts with it. But whatever their relation to polarity 

 and symmetry, the metabolic gradients are fundamental factors 

 in individuation, as the following sections will show. 



DOMINANCE AND SUBORDINATION OF PARTS IN RELATION TO THE 



AXIAL GRADIENTS 



The process of experimental reproduction in the lower animals, 

 that is, the development of new individuals or parts of individuals 

 from pieces cut from the bodies of other individuals, affords an 

 insight into the problem of individuation which cannot be obtained 

 in any other way. In many of these cases of experimental repro- 

 duction a new individuation takes place under such conditions that 

 it is possible to learn something of the manner in which it occurs. 

 A few of the more important points which have been established 

 are briefly considered here. 



Apical regions or heads may arise and develop in complete 

 independence of any other part of the body, but other levels along 

 the main axis can arise only in connection with an apical or head 

 region, or in its absence with some region representing a more 

 apical or anterior level. A few examples will make the point clear. 



In its simple, unbranched form the hydroid Tubularia consists 

 of the parts indicated in Fig. 75, at the apical end the hydranth 

 with its two sets of tentacles and the reproductive organs between 

 them, below this a long stem, and in contact with the substratum 

 a stolon. Isolated pieces of the stem more than two or three 

 millimeters in length produce a hydranth at the distal end and a 

 second hydranth may arise later at the proximal end (Fig. 76), this 

 second hydranth being the result of a reproductive process similar 

 to that occurring in this species in nature (see p. 220). But when 

 the pieces are below a certain length, which varies with different 

 regions of the body and different animals and also with different 

 external conditions, they give rise to hydranths or apical regions of 

 hydranths at one or both ends with more or less complete absence 

 of other parts. In the longer pieces of this sort a short stem may 

 be formed (Figs. 77, 78), in slightly shorter pieces single or double, 

 or more properly biaxial hydranths both complete in all respects 

 (Figs. 79, 80), or a biaxial structure like Fig. 81 with one complete 



