REMARKS ON REGENERATION 343 
system will find this behavior of the embryo in harmony 
with the ‘“biogenetic law.” 
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.’ 
In the sense of this theory, we must assume that there 
are at first present in the animal egg only specific ectoderm 
and entoderm 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, e. g., 
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 Sacus, Arbeiten des Botanischen Instituts in Wiirzburg, Vol. II, p. 457, 
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