88 Hypothesis of Structure of Germ Substance 



abnormal activation of new centres of development after 

 amputations, incisions, or in any other abnormal condi- 

 tions whatever in the most different regions of the organ- 

 ism which otherwise would have continued to constitute 

 definite somatic parts of it. 



In Planaria maculata for example these new develop- 

 mental centers of heteromorphous formations, of which 

 this animal affords perhaps the most typical cases, appear, 

 according to the results of recent researches, to be formed 

 always from one of the two ends of the piece of the 

 lateral nerve tract which is separated by the operation 

 from the other parts of this tract. 57 



Indeed it appears to be indicated by these latest and 

 most careful researches that those pieces of the planar ian 

 which contain no part of this nerve tract are not able 

 to regenerate themselves, any more than are those frag- 

 ments of infusoria which contain no part of the nucleus. 58 



Therefore this animal forms perhaps the transition 

 from those pluricellular organisms in which all the somatic 

 cells preserve throughout their capacity of regeneration 

 undiminished, to those in which this capacity exists in 

 the adult in only a very definite and special zone. 



The phenomenon which more than any other speaks 

 in favor of a nuclear somatization arising in the higher 

 multicellular organisms during development, is the cir- 

 cumstance that the capacity of regeneration diminishes 

 with age; for it is very much greater in embryos than 

 in fully developed animals. For example, if the feet of 

 an adult frog are cut off, they do not grow again, whereas 



"Charles Russell Bardeen : Factors in Heteromorphosis in Plan- 

 ariae. Arch. f. Entwicklungsmech. d. Org. Bd., XVII. i. Heft. 13 

 March 1903. P. 120, esp. P. 68; Fig. 5, 6, 7. 



"Charles Russell Bardeen : Ibid. P. 2, 3. 



