344 STUDIES IN GENERAL PHYSIOLOGY 
Planaria not all the ‘‘raw material” is consumed; yet enough 
substance for the formation of epithelium is still present in 
the leech for the repair of defects in the epithelium. 
During embryonal development the more ‘‘raw material” 
is changed into the specific organogenetic substances of the 
different organs, the larger the number of organs that are 
formed; in other words, the further the embryo progresses 
in the process of development. It is, therefore, easily intel- 
ligible why in some animals in which the ‘‘raw material” is 
present in only limited amounts regeneration is more com- 
plete in the earlier stages of the embryo than in the later 
stages. If the power of regeneration is dependent in this 
way upon the difference between the formation and consump- 
tion of the ‘“‘raw material,” it is not to be expected that the 
power of regeneration should be a function of the position of 
a species in the natural system. We can easily understand 
that this difference may be relatively great in one form, while 
in another form of the same group it drops to zero sooner or 
later during the embryonic development. We may expect, 
however, that we shall meet the latter state of affairs rela- 
tively more frequently in the more highly differentiated 
groups of Arthropods and vertebrates than in lower groups; 
which, indeed, seems to be the case. An idea of the role of 
the organogenetic substances in regeneration has been given 
by Sachs in his paper on ‘‘Stoff und Form der Pflanzenor- 
gane.” I have shown in several papers that the processes 
of regeneration, heteromorphosis and ontogenesis in animals, 
agree with the ideas of Sachs.’ 
1Part I, p.115; and On Some Facts and Principles of Physiological Morphology 
(Lectures Delivered at Woods Hole, 1893). 
Digitized by Microsoft® 
