REMARKS ON REGENERATION 343 



system will find this behavior of the embryo in harmony 

 with ihe "biogenetic law." 1 



The explanation of the fact that the embryo possesses a 

 greater capacity for regeneration than does the adult animal 

 follows in a simple way from Sachs's theory of organization. 

 Sachs assumes that the form of organs is determined by spe- 

 cific substances, and that we have just as many specific mor- 

 phogenetic substances in a plant as there are different organs 

 present in it ; but these substances are by no means all 

 preformed in the germ; they originate through chemical 

 changes from the germ substance during the process of 

 development. 



If at first only those substances which lead to the formation of 

 stems and roots are present, these, under the influence of external 

 conditions, finally give rise little by little to another category of 

 substances, which finally present themselves, in their purest form, 

 in the male and female sexual cells. We can imagine this process 

 as similar to the processes which follow one another in a chemical 

 factory, where from the original raw material chemical compounds 

 of the most varied kind gradually result, until finally the most val- 

 uable product, perhaps in an exceedingly small amount, is obtained 

 in a pure form. 1 



In the sense of this theory, we must assume that there 

 are at first present in the animal egg only specific ectoderm 

 and entoderui substances, from which, through chemical 

 changes during the process of development, such compounds 

 originate as are specific for epithelial cells, liver cells, periost 

 cells, etc. If now we ask how, according to this theory, c. //., 

 an animal which shows complete power of regeneration (Pla- 

 naria torva), differs from one which is able only to cover 

 the amputation stump with skin (Hirudo), the answer will 

 be, it seems to me, as follows: In the leech all the original 

 embryonic material (the egg substance) is used up in the 

 production of the specific organogenetic substances, while in 



1J. VON S U IIS, .Irtti-itfii '/Vs Hntiiiiixrlii-it Institute ill M"c ./;.-,/, Vol. II. p. I.'T. 



