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.5” 
Indeed it appears to be indicated by these latest and 
most careful researches that those pieces of the planarian 
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.®® 
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. 1. Heft. 13 
March 1903. P. 1—20, esp. P. 6—8; Fig. 5, 6, 7. 
**Charles Russell Bardeen: Ibid. P. 2, 3. 
