FOUNDATION OF A THEORY OF HEREDITY. 195 



liarities of an individual, can arise from the nucleoplasm of any 

 of the body-cells, a substance which, as we have just seen, has 

 lost the power of originating 1 any new kind of cell, because of the 

 continual simplification of its structure during- development? It 

 seems to me that it would be impossible for the simple nucleo- 

 plasm of the somatic cells to thus suddenly acquire the power of 

 originating the most complex nucleoplasm from which alone the 

 entire organism can be built up : I cannot see any evidence for the 

 existence of a force which could effect such a transformation. 



This difficulty has already been appreciated by other writers. 

 Nussbaum's l theoretical views, which I have already mentioned, 

 also depend upon the hypothesis that cells which have once become 

 differentiated for the performance of special functions cannot be 

 re-transformed into sexual cells : he also concludes that the latter 

 are separated from all other cells at a very early period of embry- 

 onic development, before any histological differentiation has taken 

 place. Valaoritis 2 has also recognised that the transformation of 

 histologically differentiated cells into sexual cells is impossible. 

 He was led to believe that the sexual cells of Vertebrata arise 

 from the white blood corpuscles, for he looked upon these latter 

 as differentiated to the smallest extent possible. Neither of these 

 views can be maintained. The former, because the sexual cells 

 of all plants and most animals are not, as a matter of fact, separated 

 from the somatic cells at the beginning of ontogeny; the latter, 

 because it is contradicted by the fact that the sexual cells of 

 vertebrates do not arise from blood corpuscles, but from the germinal 

 epithelium. But even if this fact had not been ascertained we 

 should be compelled to reject Valaoritis' hypothesis on theoretical 

 grounds, for it is an error to assume that white blood corpuscles 

 are undifferentiated, and that their nucleoplasm is similar to the 

 germ-plasm. There is no nucleoplasm like that of the germ- 

 cell in any of the somatic cells, and no one of these latter can be 

 said to be undifferentiated. All somatic cells possess a certain 

 degree of differentiation, which may be rigidly limited to one 

 single direction, or may take place in one of many directions. All 

 these cells are widely different from the egg-cell from which they 

 originated : they are all separated from it by many generations of 



1 M. Nussbaum, ' Archiv fur Mikroskopische Anatomic,' Bd. XVIII und XXIII. 

 " Valaoritis, 'Die Genesis des Thier-Eies.' Leipzig, 1882. 



O 2 



