THE ROUX-IVEISMANN THEORY OF DEVELOPMENT 303 



RoLix, Dc Vries, Weismann, and Hertwig; but all of them may be 

 traced back to Darwin's celebrated hypothesis of pangenesis as a 

 prototype. This hypothesis is so well known as to require but a 

 brief review. Its fundamental postulate assumes that the germ-cells 

 contain innumerable ultra-microscopic organized bodies or gcinvuilcs, 

 each of which is the germ of a cell and determines the development 

 of a similar cell during the ontogeny. The germ-cell is, therefore, in 

 Darwin's words, a microcosm formed of a host of inconceivably mi- 

 nute self-propagating organisms, every one of which predetermines 

 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 czWo.^ pang ens, are 

 contained in the nucleus, migrating thence into the cytoplasm step 

 by step during ontogeny, and thus determining the successive stages 

 of development. The same view was afterwards accepted by Hert- 

 wig and Weismann.^ 



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 ^2,2,, 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.^ 



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, 



1 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 pre-exist in tlie germ-cell, and those of the tissue-cells are derived from this 

 source in' cell-division. 



- 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. 



