409 INHERITANCE AND DEVELOPMENT 



plasm. Hereditary traits are the outcome of a definite molecular 

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

 because it contains a different idioplasm. The species is as com- 

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

 from a frog's egg 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 micella. 

 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 dynarnic properties of the micellae themselves. 

 During development the idioplasm undergoes a progressive trans- 

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

 through dynamic alterations of the conditions of tension and move- 

 ment 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 biological 

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

 concluded 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 experiments 

 of Nussbaum, Gruber, and Verworn proved that the regeneration 

 of differentiated cytoplasmic structures in the Protozoa can only 

 take place when nuclear matter is present (cf. p. 342). 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 

 gives rise to chromosomes of the same number, form, and size. Van 

 Beneden and Boveri proved (p. 182) that the paternal and maternal 

 nuclear substances are equally distributed to each of the first two 

 cells, and the more recent work of Hacker, Riickert, Herla, and 

 Zoja establishes a strong probability that this equal distribution con- 

 tinues in the later divisions. Roux pointed out the telling fact that 

 the entire complicated mechanism of mitosis seems designed to affect 



