THE IDIOPLASM THEORY 30 1 



great merit of Nageli's hypothesis to consider inheritance as effected 

 by the transmission not of a cell, considered as a whole, but of a par- 

 ticular substance, the idioplasm, contained within a cell, and forming 

 the physical basis of heredity. The idioplasm is to be sharply dis- 

 tinguished from the other constituents of the cell, which play no 

 direct part in inheritance and form a " nutritive plasma " or 

 tropJwplasm. Hereditary traits are the outcome of a definite molec- 

 ular organization of the idioplasm. The hen's egg differs from the 

 frog's because it contains a different idioplasm. The species is as 

 completely contained in the one as in the other, and the hen's egg 

 differs from a frog's as widely as a hen from a frog. 



The idioplasm was conceived as an extremely complex substance 

 consisting of elementary complexes of molecules known as micellce. 

 These are variously grouped to form units of higher orders, which, 

 as development proceeds, determine the development of the adult 

 cells, tissues, and organs. The specific peculiarities of the idioplasm 

 are therefore due to the arrangement of the micellae ; and this, in its 

 turn, is owing to dynamic properties of the micellae themselves. Dur- 

 ing development the idioplasm undergoes a progressive transforma- 

 tion of its substance, not through any material change, but through 

 dynamic alterations of the conditions of tension and movement of 

 the micellae. These changes in the idioplasm cause reactions on the 

 part of surrounding structures leading to definite chemical and plastic 

 changes, i.e. to differentiation and development. 



Nageli made no attempt to locate the idioplasm precisely or to 

 identify it with any of the known morphological constituents of the 

 cell. It was somewhat vaguely conceived as a network extending 

 through both nucleus and cytoplasm, and from cell to cell through- 

 out the entire organism. Almost immediately after the publication 

 of his theory, however, several of the foremost leaders of biologi- 

 cal investigation were led to locate the idioplasm in the nucleus, 

 and succeeding researches have rendered it more and more highly 

 probable that it is to be identified with chromatin. The grounds 

 for this conclusion, which have already been stated in Chapter 

 VII., may be here again briefly reviewed. The beautiful experi- 

 ments of Nussbaum, Gruber, and Verworn proved that the regenera- 

 tion of differentiated cytoplasmic structures in the Protozoa can only 

 take place when nuclear matter is present (cf. p. 248). The study of 

 fertilization by Hertwig, Strasburger, and Van Beneden proved that 

 in the sexual reproduction of both plants and animals the nucleus of 

 the germ is equally derived from both sexes, while the cytoplasm is 

 derived almost entirely from the female. The two germ-nuclei, which 

 by their union give rise to that of the germ, were shown by Van 

 Beneden to be of exactly the same morphological nature, since each 



