IV.] FOUNDATION OF A THEORY OF IIERFDITY. 1 89 



has also been changed in nature, and that such changes take 

 place during ontogenetic development in a regular and definite 

 manner. This view is also held by Strasburger, and it must 

 be the opinion of all who seek to derive the development of 

 inherited tendencies from the molecular structure of the germ- 

 plasm, instead of from preformed gemmules. 



We are thus led to the important question as to the forces by 

 which the determining substance or nucleoplasm is changed, 

 and as to the manner in which it changes during the course of 

 ontogeny, and on the answer to this question our further con- 

 clusions must depend. The simplest hypothesis would be to 

 suppose that, at each division of the nucleus, its specific sub- 

 stance divides into two halves of unequal quality, so that the 

 cell-bodies would also be transformed ; for we have seen that 

 the character of a cell is determined by that of its nucleus. 

 Thus in any Metazoon the first two segmentation spheres 

 would be transformed in such a manner that one only con- 

 tained the hereditary tendencies of the endoderm and the 

 other those of the ectoderm, and therefore, at a later stage, the 

 cells of the endoderm would arise from the one and those of 

 the ectoderm from the other ; and this is actually known to 

 occur. In the course of further division the nucleoplasm of 

 the first ectoderm cell would again divide unequally, e. g. into 

 the nucleoplasm containing the hereditary tendencies of the 

 nervous system, and into that containing the tendencies of the 

 external skin. But even then, the end of the unequal division 

 of nuclei would not have been nearly reached ; for, in the 

 formation of the nervous system, the nuclear substance which 

 contains the hereditary tendencies of the sense-organs would, 

 in the course of further cell-division, be separated from that 

 which contains the tendencies of the central organs, and the 

 same process would continue in the formation of all single 

 organs, and in the final development of the most minute histo- 

 logical elements. This process would take place in a definitely 

 ordered course, exactly as it has taken place throughout a verj* 

 long series of ancestors ; and the determining and directing 

 factor is simply and solely the nuclear substance, the nucleo- 

 plasm, which possesses such a molecular structure in the 

 germ-cell that all such succeeding stages of its molecular 

 structure in future nuclei must necessarily arise from it, as 



