CONCLUSIONS AND SUGGESTIONS 191 



and progresses from this to other parts. Animal metab- 

 olism evidently synthesizes highly stable molecules, even 

 where metabolic activity is most intense. 



In the plant the whole substratum may apparently 

 be mobilized to some extent when the metabolic rate 

 is high, and only as the rate becomes lower do substances 

 accumulate as structure. In nearly all if not all animals, 

 on the other hand, certain protoplasmic substances are 

 relatively more stable under the existing metabolic 

 conditions than in the plant and therefore accumulate, 

 and a progressive structural development and differ- 

 entiation occur even when the metabolic rate is highest. 

 In the animals the morphological structure which de- 

 velops in the region of highest metabolic rate is physio- 

 logically the most stable structure of the body, because 

 the less stable substances are decomposed in the intense 

 metabolic activity and so do not form permanent con- 

 stituents of the substratum. In regions of lower meta- 

 bolic rate substances accumulate which are readily 

 removed by an increase in metabolic rate. These parts 

 may therefore undergo dedifferentiation and rediffer- 

 entiation. The head-region, however, or, more specifi- 

 cally, the central nervous system, is almost or quite 

 incapable of dedifferentiation under ordinary conditions, 

 because its structure has developed under conditions 

 of more intense metabolic activity than any other 

 part of the body and is therefore more stable. If the 

 metabolic rate could be increased sufficiently above the 

 rate in the developing nervous system without bringing 

 about death, doubtless dedifferentiation of the nervous 

 system would occur to some extent. To refer briefly 

 to the analogy between the organism and the flowing 



