The germ-plasm : a theory of heredity / [A. Weismann] ; translated by W. Newton Parker and Harriet Rönnfeldt.

  • Weismann, August, 1834-1914.
Date:
1893
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The germ-plasm : a theory of heredity / [A. Weismann] ; translated by W. Newton Parker and Harriet Rönnfeldt. Source: Wellcome Collection.

    62/522 (page 32)
    32 THE GERM-PLASM. the oftspring in such a iiianner as to cause them to resemble the cells of the paternal body. The chromatin is therefore in a condition to impress the specific character on the cell in the nucleus of which it is contained. As the thousands of cells which constitute an organism pos¬ sess very different properties, the chroinatin which controls them cannot be uniform ; it must be different in each kind of cell. The chromatin, moreover, cannot Ьесо7пе different in the cells of the fully formed organism ; the differences in the chromatin controlling the cells must begin with the development of the egg- cell, and must increase as development proceeds ; for otherwise the different products of the division of the ovum could not give rise to entirely different hereditary tendencies. This is, how¬ ever, the case. Even the two first daughter-cells which result from the division of the egg-cell give rise in many animals to totally different parts. One of them, by continued cell division, forms the outer germinal layer, and eventually all the organs which arise from it,—e.g., the epidermis, central nervous system, and sensory cells ; the other gives rise to the inner germinal layer and the organs derived from it,—the alimentary system, certain glands, &c. The conclusion is inevitable that the chromatin determining these hereditary tendencies is different in the daughter-cells. This holds good in all subsequent stages of ontogeny; the difference between the developmental tendencies of the cells resulting from the division of the ovum is in exact proportion to that between the chromatin substance of their nuclei. Ontogeny, or the developme7tt of the individual, depends therefore on a series of gradual qualitative changes in the miclear substance of the egg-cell. The fundamental principle of the view which has just been briefly sketched was put forward by me some years ago, and I then made use of the term idioplasm to represent the substance which is contained in the chromatin bodies of the nucleus, and which detemiines the nature of the whole cell. Oscar Hertwig also independently adopted this term, which had first been intro¬ duced by Nägeli with a somewhat different meaning. As stated in the first section of this book, Nägeli 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