Hertwig 233 
termediary through which must pass all the characters 
newly acquired by the soma and thanks to which they 
become transformed so that they can then be inherited by 
following generations. 
Oscar Hertwig 
Oscar Hertwig gives in the following words the es- 
sentials of his theory of biogenesis. 
“The cells, necessarily equal specifically on account of 
their origin in the segmentation of the egg, which are 
combined to constitute an organic system of a higher 
order, have their character determined by the relations in 
which they become placed during the course of develop- 
ment. Their state becomes modified when these relations 
are modified. For the cell organism is an extremely ir- 
ritable substance so that the slightest influences are suff- 
cient to bring about modifications in it.” 
“Contrary to the mosaic theory of Roux and the germ 
plasm theory of Weismann, the theory of biogenesis is 
based upon the principle, that from the commencement of 
development, the cells arising from the segmentation of 
the egg are constantly in the most intimate relation with 
one another, and the character of the developmental pro- 
cess is determined essentially by this fact. The cells do 
not take their especial future character of their own initia- 
tive, but their character becomes determined according to 
laws which result from the co-operative action of all the 
cells during the successive stages of development of the 
entire organism.” 
“The relations of the rapidly multiplying cells of the 
substance capable of development are constantly chang- 
ing in accordance with general laws, and the relations 
between these internal factors and those which are with- 
