1 84 CONTINUITY 01 THE GERM-PLASM AS THE [IV. 



indeed already spoken of a ' cyto-idioplasm,' and it is certainly 

 obvious that the cell-body often possesses a specific character, 

 but we must in all cases assume that such a character is 

 impressed upon it by the influence of the nucleus, or, in other 

 words, that the direction in which the cell-substance is differ- 

 entiated in the course of development is determined by the 

 quality of its nuclear substance. So far, therefore, the deter- 

 mining nuclear substance corresponds to the idioplasm alone, 

 while the substance of the cell-body must be identified with the 

 somato-plasm (' Nahrplasma') of Nageli. At all events, in 

 practice, it will be well to restrict the term idioplasm to the 

 regulative nuclear substance alone, if we desire to retain the 

 well-chosen terms of Nageli's theory. 



But the second part of Nageli's theory of the idioplasm is 

 also untenable. It is impossible that this substance can have 

 the same constitution everywhere in the organism and during 

 every stage of its ontogeny. If this were so, how could the 

 idioplasm effect the great differences which obtain in the for- 

 mation of the various parts of the organism ? In some passages 

 of his work Nageli seems to express the same opinion ; e.g. on 

 page 31 he says, ' It would be practicable to regard — although 

 only in a metaphorical sense — the idioplasms of the different 

 cells of an individual as themselves different, inasmuch as they 

 possess specific powers of production : we should thus include 

 among these idioplasms all the conditions of the organism which 

 bring about the display of specific activity on the part of cells.' 

 It can be clearly seen from the passages immediately preceding 

 and succeeding the above-quoted sentence, that Nageli, in 

 speaking of these changes in the idioplasm, does not refer to 

 material, but only to dynamical changes. On page 53 he lays 

 special stress upon the statement that 'the idioplasm during 

 its growth retains its specific constitution everywhere through- 

 out the organism,' and it is only ' within these fixed structural 

 limits that it changes its conditions of tension and movement, 

 and thus alters the forms of growth and activity which are 

 possible at each time and place. Against such an interpretation 

 weighty objections can be raised. At present I will only men- 

 tion that the meaning of the phrase ' conditions of tension and 

 movement ' ought to be made clear, and that we ought to be 

 informed how it is that mere differences in tension can produce 



