344 STUDIES IN GENERAL PHYS-IOLOGY 



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. 1 



1 Part I, p. 115 ; and On Some Facts and Principles of Physiological Morphology 

 (Lectures Delivered at Woods Hole, 



