IV.] FOUNDATION OF A THEORY OF HEREDITY. I 79 



which must be given up in the other cases; although such 

 advantage might consist in assisting the development of the 

 fertilized ovum and not in any increase of the true fertilizing 

 substance. At the present time we are indeed disposed to 

 recognize this advantage in still more unimportant matters, 

 but at that time the ascertained facts did not justify us in the 

 assertion that fertilization is a mere fusion of nuclei, and M. 

 Nussbaum ^ quite correctly expressed the state of our know- 

 ledge when he said that the act of fertilization consisted in 

 ' the union of identical parts of two homologous cells.' 



Pfluger's discovery of the ' isotropism ' of the ovum was the 

 first fact which distinctly pointed to the conclusion that the 

 bodies of the germ-cells have no share in the transmission of 

 hereditary tendencies. He showed that segmentation can be 

 started in different parts of the body of the &%g^ if the latter 

 be permanently removed from its natural position. This 

 discovery constituted an important proof that the body of the 

 0^^^% consists of a uniform substance, and that certain parts or 

 organs of the embryo cannot be potentially contained in certain 

 parts of the ^%g, so that they can only arise from these re- 

 spective parts and from no others. Pfluger was mistaken in 

 the further interpretation, from which he concluded that the 

 fertilized ovum has no essential relation to the organization of 

 the animal subsequently formed by it, and that it is only the 

 recurrence of the same external conditions which causes the 

 germ-cell to develope always in the same manner. The force 

 of gravity was the first factor, which, as Pfluger thought, de- 

 termined the building up of the embryo : but he overloQked 

 the fact that isotropism can only be referred to the body of 

 the ^g%^ and that besides this cell-body there is also a nucleus 

 present, from which it was at least possible that regulative 

 influences might emanate. Upon this p^oint Born ^ first showed 

 that the position of the nucleus is changed in eggs which are 

 thus placed in unnatural conditions, and he proved that the 

 nucleus must contain a principle which in the first place 

 directs the formation of the embryo. Roux'' further showed 

 that, even when the eifect of gravity is compensated, the 



^ Arch. f. mikr. Anat., Bd. 23. p. 182, 1884. 



^ Born, ' Biologische Untersuchungen,' I, Arch. Mikr. Anat., Bd. XXIV. 



^ Roux, * Beitrage zum Entwicklungsmechanismus des Embryo,' 1884. 



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