Her twig 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 suffi- 

 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- 



