80 NATIONAL TRENDS IN BIOLOGY 



than Child's hypothesis of metabolic gradients. It has 

 captured the imagination of the younger generation of 

 zoologists, and is exercising an increasing influence upon 

 them." 



To understand the theory, it is essential to know some 

 of the observations which led to its formulation. In anes- 

 thetizing animals, the head region is always more sus- 

 ceptible to a retardation of metabolic activity than parts 

 some distance from it. When an animal is killed by high 

 concentrations of anesthetics, the head always dies first. 

 In regeneration experiments, where a flatworm, for ex- 

 ample, is cut transversely into many pieces, practically 

 all of the cut pieces, with the exception of the head, will 

 grow a new head from the anterior end. This shows that 

 each cut piece had the potentiality of developing one or 

 more heads, but was prevented, while it was part of the 

 whole, by the inhibitory and regulative action of the 

 original head. 



From these and many other experiments, scientists 

 have accepted as a fact that not only are the rate and 

 the intensity of chemical changes which take place in a 

 growing, living thing different in different parts of the 

 body, being greatest in the head region and gradually 

 becoming less as the distance from the head region be- 

 comes greater, but that the head region dominates the 

 rest of the body. That the latter part of this statement 

 is true is demonstrated by such experiments as the fol- 

 lowing. A piece of tissue quite foreign to another struc- 



