404 



INHERITANCE AND DEVELOPMENT 



the formation of one of the adult cells. De Vries ('89) brought this 

 conception into relation with the theory of nuclear idioplasm by 

 assuming that the gemmules of Darwin, which he called pangens, are 

 contained in the nucleus, migrating thence into the cytoplasm step 

 by step during ontogeny, and thus determining the successive stages 

 of development. The hypothesis is further modified by the assump- 

 tion that the pangens are not cell-germs, as Darwin assumed, but 

 ultimate protoplasmic units of which cells are built, and which are 

 the bearers of particular hereditary qualities. The same view was 

 afterward accepted by Hertwig and Weismann. 2 



The theory of germinal localization is thus transferred from the 

 cytoplasm to the nucleus. It is not denied that the egg-cytoplasm 

 may be more or less distinctly differentiated into regions that have a 

 constant relation to the parts of the embryo. This differentiation is, 

 however, conceived, not as a primordial characteristic of the egg, but 

 as one secondarily determined through the influence of the nucleus. 

 Both De Vries and Weismann assume, in fact, that the entire cyto- 

 plasm is a product of the nucleus, being composed of pangens that 

 migrate out from the latter, and by their active growth and multipli- 

 cation build up the cytoplasmic substance. 3 



D. THE ROUX-WEISMANN THEORY OF DEVELOPMENT 



We now proceed to an examination of two sharply opposing hy- 

 potheses 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-propa- 

 gating units in the idioplasm. This hypothesis may therefore be laid 



1 Cf. p. 290. 



2 The neo-pangenesis of De Vries differs from Darwin's hypothesis in one very important 

 respect. Darwin assumed that the gemmules arose in the body, being thrown off as germs 

 by the individual tissue-cells, transported to the germ-cells, and there accumulated as in a 

 reservoir; and he thus endeavoured to explain the transmission of acquired characters. De 

 Vries, on the other hand, denies such a transportal from cell to cell, maintaining that the 

 pangens arise or preexist in the germ-cell, and those of the tissue-cells are derived from this 

 source by cell-division. 



3 This conception obviously harmonizes with the role of the nucleus in the synthetic 

 process. In accepting the view that the nuclear control of the cell is effected by an emana- 

 tion of specific substances from the nucleus, we need not, however, necessarily adopt the 

 pangen-hypothesis. 



