198 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



If we must therefore assume that the molecular structure of 

 the nucleoplasm becomes simpler in the course of ontogeny, as 

 the number of structures which it potentially contains become 

 smaller, it follows that the nucleoplasm in the cells of fully 

 differentiated tissues — such as muscle, nerve, sense-organs, or 

 glands — must possess relatively the most simple molecular 

 structure; for it cannot originate any fresh modification of 

 nucleoplasm, but can only continue to produce cells of the 

 same structure, although it does not alwaj's retain this power. 



We are thus brought back to the fundamental question as to 

 how the germ-cells arise in the organism. Is it possible that 

 the nucleoplasm of the germ-cell, with its immensely complex 

 molecular structure, potentially containing all the specific pecu- 

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

 of the bodj'-cells, — a substance which, as we have just seen, 

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

 of the continual simplification of its structure during develop- 

 ment ? It seems to me that it would be impossible for the 

 simple nucleoplasm of the somatic cells to thus suddenly ac- 

 quire 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' 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 embryonic development, before any histological dif- 

 ferentiation has taken place. Valaoritis^ has also recognized 

 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 cor- 

 puscles, 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 



' M. Nussbaum. 'Archiv fur Mikroskopische Anatomic,' Bd. XVIII 

 und XXIII. 



^ Valaoritis, * Die Genesis des Thicr-Eics.' Leipzig, 1882. 



