204 SENESCENCE AND REJUVENESCENCE 



organisms in both plants and animals. At present, however, it 

 is possible to call attention only very briefly to some of these. It 

 is, for example, a well-known fact that in those plants which possess 

 a definite physiological and morphological axis or axes the apical 

 region of the axis is the region of highest rate of metabolism, and a 

 more or less definite downward gradient in rate exists along the 

 axis, at least for a certain distance from the apical region. This 

 gradient appears in the rate of growth at various levels of the axis, 

 in the precedence in development of the lateral buds near the apical 

 end when the chief growing tip has been removed, and in many 

 other features of plant life, but the question of its significance has 

 received little attention. 



As regards animals, the so-called law of antero-posterior devel- 

 opment indicates the existence of a metabolic gradient along the 

 main axis of the organism during embryonic development. This 

 "law" is merely the statement of the observed fact of embryology 

 that in general the first parts to become morphologically visible 

 are the apical or anterior regions, and these are followed in sequence 

 by successively more posterior or basal parts. In other words, that 

 region of the egg or early embryo which has the highest rate of 

 metabolism gives rise to the apical or head-region, which, in conse- 

 quence of the higher rate, becomes differentiated in advance of other 

 parts, and these follow in sequence along the axis. This fact of 

 embryology is familiar to every zoologist, and its significance as the 

 expression of a gradient in dynamic activity along the axis cannot 

 be doubted, although, so far as I am aware, no one has called atten- 

 tion to it. 



Moreover, other facts of animal embryology indicate very 

 clearly the existence of symmetry gradients. In the bilaterally 

 symmetrical invertebrates, with ventral nerve cord, including most 

 worms and the arthropods, and particularly in those forms where 

 the egg contains much yolk so that the embryo is more or less spread 

 out upon it, the ventral and median regions of the embryo at any 

 given level of the body develop more or less in advance of the dorsal 

 and lateral regions. In such forms the regions which give rise to 

 ventral and median parts must have a higher rate of metabolism 

 than those which give rise to dorsal and lateral parts. 



