THE ROUX-WEISMANN THEORY OF DEVELOPMENT 305 



by its own nature, and such transformation is accompanied by a 

 gradual change in the character of the cell-bodies." ^ In later writ- 

 ings Weismann advanced far beyond this, building up an elaborate 

 artificial system, which appears in its final form in the remarkable 

 book on the germ-plasm ('92). Accepting De Vries's conception of 

 the pangens, he assumes a definite grouping of these bodies in the 

 germ-plasm or idioplasm (chromatin), somewhat as in Nageli's concep- 

 tion. The pangens or biopJiorcs are conceived to be successively ag- 

 gregated in larger and larger groups ; namely, (i) dcteinniiiants, which 

 are still beyond the limits of microscopical vision ; (2) ids, which are 

 identified with the visible chromatin-granules ; and (3) idants, or 

 chromosomes. The chromatin has, therefore, a highly complex fixed 

 architecture, which is transmitted from generation to generation, and 

 determines the development of the embryo in a definite and specific 

 manner. Mitotic division is conceived as an apparatus which may 

 distribute the elements of the chromatin to the daughter-nuclei either 

 equally or unequally. In the former case {'' lioiiiccokinesis,'" integral 

 or quantitative division), the resulting nuclei remain precisely equiva- 

 lent. In the second case {'' /wtcrokinesis,'' qualitative or dijfcrential 

 division), the daughter-cells receive different groups of chromatin- 

 elements, and hence become differently modified. During ontogeny, 

 through successive qualitative divisions, the elements of the idioplasm 

 or genn-plasni (chromatin) are gradually sifted apart, and distributed 

 in a definite and predetermined manner to the various parts of the 

 body. " Ontogeny depends on a gradual process of disintegration of 

 the id of germ-plasm, which splits into smaller and smaller groups of 

 determinants in the development of each individual. . . . Finally, 

 if we neglect possible complications, only 07ie kind of determinant re- 

 mains in each cell, viz. that which has to control that particular cell or 

 group of cells. ... In this cell it breaks up into its constituent bi- 

 ophores, and gives the cell its inherited specific character."'-^ Devel- 

 opment is, therefore, essentially evolutionary and not epigenetic ; '^ its 

 point of departure is a substance in which all of the adult characters 

 are represented by preformed, prearranged germs; its course is the 

 result of a predetermined harmony in the succession of the qualitative 

 divisions by which the hereditary substance is progressively disinte- 

 grated. In order to account for heredity through successive genera- 

 tions, Weismann is obliged to assume that, by means of quantitative 

 or integral division, a certain part of the original germ-plasm is car- 

 ried on unchanged, and is finally delivered, with its original architecture 

 unaltered, to the germ-nuclei. The power of regeneration is explained, 

 in like manner, as the result of a transmission of unmodified or slightly 

 modified germ-plasm to those parts capable of regeneration. 



1 Essay IV., p. 193, 1885. - Gerni-plasin, pp. 76, 77. -^ I.e., p. 15. 



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