186 . THE CONTINUITY OF THE GERM-PLASM AS THE 



in a regular and definite manner. This view is also held by Stras- 

 burger, 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 onto- 

 geny, and on the answer to .this question our further conclusions 

 must depend. The simplest hypothesis would be to suppose that, 

 at each division of the nucleus, its specific substance 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 contained 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 histological elements. This process would take place 

 in a definitely ordered course, exactly as it has taken place through- 

 out a very long series of ancestors ; and the determining and 

 directing factor is simply and solely the nuclear substance, the 

 nucleoplasm, 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 soon as the re- 

 quisite external conditions are present. This is almost the same 

 conception of ontogenetic development as that which has been held 

 by embryologists who have not accepted the doctrine of evolution : 



