134 THE GERM-PLASM 



new individuals.* The stimulus to proliferation, as in the case of 

 regeneration in animals, is due to the removal of the opposition to 

 growth ; the cells must, however, be adapted for this reaction, other- 

 wise the proliferation cannot take place : the stems as well as the 

 roots and veins of herbaceous plants do not by any means always 

 respond to an injury by the formation of callus. This process is 

 therefore not a primary quality of the plant, but an adaptation, 

 due, in my opinion, to the association of certain supplementary 

 determinants with the active idioplasm of certain kinds of cells. 

 The formation of callus is probably the only process in plants 

 which can be regarded as an actual regeneration. 



5. Regeneration in Animal Embryos, and the 

 Principles of Ontogeny 



\ The theory of heredity which has now been formulated, — and 

 more especially that portion of it which concerns the composition 

 of the germ-plasm out of determinants, and the gradual disin- 

 tegration of the mass of determinants in the germ-plasm during 

 the course of ontogeny, — is based on the assumption that the 

 cells control themselves : that is to say, the fate of the cells is 

 determined by forces situated within them, and not by external 

 influences. The primary cells of the ectoderm and of the endo- 

 derm arise by the division of the fertilised egg-cell and its con- 

 tained germ-plasm, because the determinants of the ectoderm 

 are passed into one cell and those of the endoderm into the 

 other, and not because some external influence, such as the 

 force of gravity, affects the cells in a different manner. Simi- 

 larly a certain cell in a subsequent embryonic stage does not 

 give rise to a nerve-, a muscle-, or an epithelial-cell because it 

 happens to be so situated as to be influenced by certain other 

 cells in one way or another, but because it contains special 

 determinants for nerve-, muscle-, or epithelial-cells. 



This conception of the predestination of the individual cells, 

 the fate of which, together with that of their successors, is deter- 

 mined by the idioplasm they contain, was first imperfectly ex- 

 pressed in the theory formerly propounded by His,t in which he 



* J. Sachs, ' Lectures on the Physiology of Plants," Leipzig, 1882, p. 709. 

 (English edition, translated by H. Marshall Ward, O.xford, 1887.) 



t Wilhelm His, ' Unsre Korperform u. das physiologische Problem ihrer 

 Entstehung," Leipzig, 1874. 



