424 C. JUDSON HERRICK 



tissues it follows that every living body exhibits a metabolic 

 gradient. There is in most plants and animals a primary axis 

 with a high rate at the apical or head end and a diminishing 

 rate as we pass away from this center. There are also various 

 subsidiary metabolic gradients in addition to this primary axial 

 gradient, the details of which we need not here enter upon. 



7. The point of highest metabolic rate is a " center of domin- 

 ance " in the sense that the physiological activities of all other 

 parts of the body are to some extent regulated and integrated 

 from this dominant center of highest metabolic rate. In all 

 higher animals this center of dominance is in the brain and the 

 chief mechanism of regulatory control is the central and per- 

 ipheral nervous system. The nervous system, however, con- 

 tributes nothing new in principle to what is found in plants and 

 the lowest animals possessing no nervous organs whatever. 



8. In organisms which lack highly differentiated nervous 

 systems the controlling and integrating influence exercised by 

 the center of dominance over other parts of the body diminishes 

 with the distance of the parts from this center; and when by the 

 growth of the body this distance exceeds a certain maximum 

 the influence of the center of dominance is so reduced as to result 

 in a " physiological isolation " of part of the body from this 

 influence. In this case a secondary center of dominance may 

 arise, subsidiary to which a complete new individual is consti- 

 tuted. This is the mechanism of asexual reproduction by fis- 

 sion and budding in plants and many lower animals. The 

 regeneration of lost parts in invertebrates and lower vertebrates 

 is possible by virtue of a similar but less complete physiological 

 isolation of the region of injury. In higher animals, where the 

 nervous system provides a more efficient apparatus of control 

 from the primary center of dominance these phenomena of 

 fission and local regeneration are not found, although even in 

 man the phenomena of healing of wounds exhibit some small 

 measure of physiological isolation and local control. 



9. The characteristics of the center of dominance are dependent 

 primarily upon its higher rate of metabolism-. A high rate of 

 metabolism may be effected by one of two types of mechanism, 

 either (1) by relatively undifferentiated protoplasm, all of whose 

 substance is relatively unstable and so capable of rapid chemical 

 transformation (the " young " type of tissue), or (2) by organs 



