THE IDIOPLASM THEORY We owe to Nageli the first systematic attempt to discuss heredity regarded as inherent in a definite physical basis ; 3 but it is hardly necessary to point out his great debt to earlier writers, foremost among them Darwin, Herbert Spencer, and Hackel. It was the 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 particular substance, the idioplasm, contained within a cell, and forming the physical basis of heredity. The idioplasm is to be sharply distinguished from the other constituents of the cell, which play no direct part in inheritance and form a " nutritive plasma " .or trophoplasm. 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 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 micelle. 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 arc therefore due to the arrangement of the micellae ; and this, in its turn, is owing to dynamic properties of the micellw themselves. During development the idioplasm undergoes a progressive transformation 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 throughout 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 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 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. 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 gives rise to chromosomes of the same number, form, and size. Van Beneden and Boveri proved (p. 134) that the paternal and maternal nuclear substances are equally distributed to each of the first two cells, and the more recent work of Hacker, Ruckert, Herla, and Zoja establishes a strong probability that this equal distribution continues in the later divisions. Roux pointed out the telling fact that the entire complicated mechanism of mitosis seems designed to effect the most accurate division of the entire nuclear substance in all of its parts, while fission of the cytoplasmic cell-body is in the main a mass-division, and not a meristic division of the individual parts. Again, the complicated processes of maturation show the significant fact that while the greatest pains is taken to prepare the germ-nuclei for their coming union, by rendering them exactly equivalent, the cytoplasm becomes widely different in the two germ-cells and is devoted to entirely different functions.
It was in the main these considerations that led Hertwig, Strasburger, Kolliker, and Weismann independently and almost simultaneously to the conclusion that the nucleus contains the physical basis of inheritance, and that chromatin, its essential constituent, is the idioplasm postulated in Nageli's theory. This conclusion is now widely accepted ; and notwithstanding certain facts which at first sight may seem opposed to it, I believe it rests upon a basis so firm that it may be taken as one of the elementary data of heredity. To accept it is, however, to reject the theory of germinal localization in so far as it assumes a pre-localization of the egg-cytoplasm as a fundamental character of the egg. For if the specific character of the organism be determined by an idioplasm contained in the chromatin, then every characteristic of the cytoplasm must in the long run be determined from the same source. A striking illustration of this fact is given by the phenomena of colour-inheritance in plant-hybrids, as De Vries has pointed out. Pigment is developed in the embryonic cytoplasm, which is derived from the mother-cell ; yet in hybrids it may be inherited from the male through the nucleus of the germ-cell. The specific form of cytoplasmic metabolism by which the pigment is formed must therefore be determined by the paternal chromatin in the germ-nucleus, and not by a predetermination of the egg-cytoplasm.