INTRODUCTION 33 



duced by Nageli with a somewhat different meaning. As stated 

 in the first section of this book, Nageh defined idioplasm as the 

 guiding and controlling substance of the body, in contrast to the 

 more passive and controllable trophoplasm. It is open to doubt 

 whether the latter term should be retained, but the former is 

 certainly happily chosen. It is true Nageli did not mean to 

 indicate any definite substance visible under the microscope when 

 he used the word idioplasm, for the facts of nuclear division and 

 fertilisation were then unknown. But these facts are so convincing 

 that no doubt as to what is to be regarded as the idioplasm is any 

 longer possible, and Nageli's conception of an idioplasm forming 

 a network, traversing and connecting the contents of all, the 

 cells in the organism, may be regarded as abandoned. We are 

 therefore justified in transferring the term introduced by him to 

 the nuclear substance which determines the nature of the cell. 



We now therefore understand by the term idioplasm the 

 nuclear substance controlling any particular cell. This is at the 

 same time the hereditary substance, for it is never formed afresh, 

 but is always derived from the idioplasm of another cell; more- 

 over, it not only determines the actual characters of the partic- 

 ular cell, but also those of all of its descendants. 



Hence we must assume a difference in the idioplasm not only 

 in dealing with two cells differing in structure and functions, 

 but also in all cases in which we know that different primary 

 constituents are contained in two cells. This has often been 

 overlooked in using the term ' embryonic cells ' merely in the 

 sense of equivalent elements ' which may give rise to any parts,^ 

 simply because they frequently resemble one another, assuming 

 that they must therefore always be actually equivalent. It is 

 quite true that the idioplasm of such cells appears similar, at 

 least we can recognise no definable differences in the chromatin 

 rods of two cells in the same animal. But this is no argument, 

 against the assumption of an internal difference. The perfect 

 external resemblance between two eggs is not a sufficient reason 

 why two identical chickens should be hatched from them. The 

 eggs may have been produced by different mothers, or they 

 may have been fertilised by two different males. We cannot 

 perceive these slight differences in either case, and we could 

 not even do so by attempting to analyse the idioplasm concealed 

 in the nuclei of the two eggs by the aid of our most powerful 

 objectives. Theoretical considerations will show later on that 



