Sept. 1894- ''PREFORMATION OR NEW FORMATION." 185 



nucleus is the bearer of the hereditary plasm, and may be inferred to 

 be more stable than protoplasm and less subject to outer influences. It 

 is the inherited plasm of the nucleus that stamps the specific character 

 of the organism. The capacity of the cell to feed, grow, and divide 

 is the mechanism by which the continuity of life is secured. 



The cell plays a leading part in Weismann's theory. His germ- 

 plasm is made up of primary units, the biophores, which are built up 

 into determinants, a determinant existing for each independently 

 varying cell or group of cells in the adult organism. 



The arrangement of the biophores in the determinants, and of 

 these in the ids or higher units, follow an elaborate historical archi- 

 tecture. This complicated germ-plasm lies in the nucleus, and nuclear 

 division is the mechanism by which it passes from cell to cell. An 

 individual life-history begins with a fertilised egg-cell containing 

 paternal and maternal germ-plasm, which have come respectively in 

 the nucleus of the ovum and in the head of the spermatozoon. In the 

 actual course of ontogeny two things happen. By a series of nuclear 

 divisions some of the germ-plasm is saved in an unaltered form, and 

 ultimately gives rise to the sexual cells of the new individual. The 

 structure of the rest of the germ-plasm in the coarse of another series 

 of nuclear divisions gradually breaks down in such an orderly 

 fashion, that the various determinants are marshalled to their 

 proper places until ultimately the tissue cells of the various 

 parts of the embryo and the adult, instead of containing a compli- 

 cated plasm, contain only the determinants of their own order. 

 Thus nerve-cells contain only the determinants of nerve-cells, 

 muscle-cells of muscle-cells, and so forth. In certain special cases, 

 which Weismann believes to be secondary adaptations, in addition to 

 the specific determinants, cells may contain some other latent deter- 

 minants, to provide for the regeneration of lost parts and so forth. 

 But, with these exceptions, there is in every organism an impassable 

 separation between the unaltered germ-plasm, which has been pre- 

 served to pass into the sex-cells, and the specialised simplified nuclear 

 plasm of the various independently varying parts of the organism. 

 Moreover, inasmuch as the unaltered germ-plasm of the sexual cells 

 is continuous from generation to generation, while the part of it which 

 breaks up into the nuclear matter of the various tissue constituents 

 perishes with these at the death of the organism, Weismann dis- 

 tinguishes the one as the immortal germ-plasm, the other as the 

 mortal somatic plasm. 



Comparing this theory with actual knowledge of cells, Hertwig 

 points out that it implies two quite different kinds of nuclear 

 division. That kind of nuclear division by which the unaltered 

 germ-plasm is handed on from cell to cell until it reaches the new 

 sex-cells may be called " heirs-equal " division (erbgleiche Theilung), 

 for in it the nuclear matter is distributed equally and unaltered between 

 the two daughter-cells. That kind of nuclear division which occurs 



