THE ROUX-WEISMANN THEORY OF DEVELOPMENT We now proceed to an examination of two sharply opposing hypotheses of development based on the theory of nuclear idioplasm. One of these originated with Roux ('83) and has been elaborated especially by Weismann. The other was clearly outlined by De Vries ('89), and has been developed in various directions by Oscar Hertwig, Driesch, and other writers. In discussing them, it should be borne in mind that, although both have been especially developed by the advocates of the pangen-hypothesis, neither necessarily involves that hypothesis in its strict form, i.e. the postulate of discrete self-propagating units in the idioplasm. This hypothesis may therefore be laid aside as an open question, and will be considered only in so far as it is necessary to a presentation of the views of individual writers.
The Roux-Weismann hypothesis has already been touched on at p. 183. Roux conceived the idioplasm (i.e. the chromatin) not as a single chemical compound or a homogeneous mass of molecules, but as a highly complex mixture of different substances, representing different qualities, and having their seat in the individual chromatingranules. In mitosis these become arranged in a linear series to form the spireme-thread, and hence may be precisely divided by the splitting of the thread. Roux assumes, as a fundamental postulate, that division of the granules may be either quantitative or qualitative. In the first mode the group of qualities represented in the mothergranule is first doubled and then split into equivalent daughter-groups, the daughter-cells therefore receiving the same qualities and remaining of the same nature. In " qualitative division," on the other hand, the mother-group of qualities is split into dissimilar groups, which, passing into the respective daughter-nuclei, lead to a corresponding differentiation in the daughter-cells. By qualitative divisions, occurring in a fixed and predetermined order, the idioplasm is thus split up during ontogeny into its constituent qualities, which are, as it were, sifted apart and distributed to the various nuclei of the embryo. Every cell-nucleus, therefore, receives a specific fonn of chromatin which determines the nature of the cell at a given period and its later history. Every cell is thus endowed with a power of self-determination, which lies in the specific structure of its nucleus, and its course of development is only in a minor degree capable of modification through the relation of the cell to its fellows (" correlative differentiation ").
Roux's hypothesis, be it observed, does not commit him to the theory of pangenesis. It was reserved for Weismann to develop the hypothesis of qualitative division in terms of the pangen-hypothesis, and to elaborate it as a complete theory of development. In his first essay ('85), published before De Vries's paper, he went no further than Roux. "I believe that we must accept the hypothesis that in indirect nuclear division, the formation of non-equivalent halves may take place quite as readily as the formation of equivalent halves, and that the equivalence or non-equivalence of the subsequently produced daughter-cells must depend upon that of the nuclei. Thus,
during ontogeny a gradual transformation of the nuclear substance takes place, necessarily imposed upon it, according to certain laws, by its own nature, and such transformation is accompanied by a gradual change in the character of the cell-bodies." 1 In later writings 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 conception. The pangens or biophores are conceived to be successively aggregated in larger and larger groups ; namely, (I) determinants, 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 ("homaokinesis," integral or quantitative division), the resulting nuclei remain precisely equivalent. In the second case ("heterokinesis," qualitative or differential division), the daughter-cells receive different groups of chromatinelements, and hence become differently modified. During ontogeny, through successive qualitative divisions, the elements of the idioplasm or germ-plasm (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 one kind of determinant remains 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 biophores, and gives the cell its inherited specific character." 2 Development is, therefore, essentially evolutionary and not epigenetic ; 3 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 disintegrated. In order to account for heredity through successive generations, Weismann is obliged to assume that, by means of quantitative or integral division, a certain part of the original germ-plasm is carried 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.