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.

    88/522 (page 58)
    58 THE GERM-PLASM. and van Rees have since shown that the process takes place as follows :—the formation of portions of the new alimentary canal begins in certain cells which are separated by fairly regular intervals; these then spread until they come into contact with one another. The idioplasm of the new in¬ testinal cells is consequently only contained in these formative cells, and it is natural to suppose that each of them contains only one kind of determinant. The same appears to be the case with the hair of mammals. Every hair does not possess a special determinant in the germ, but more or less extensive regions of the hairy covering are represented each by one determinant. These regions are not large, as is shown by the stripes and spots on the coat of such animals as the tiger and leopard. The recurrence in the son, on exactly the same part of the head as in the parent, of an abnormal tuft of white hair, has been observed in the human subject. Similar hereditary parts or determinates may be observed in butterflies, in which the colours on the wings often form very complicated lines and spots of slight extent but of great constancy. Such regions are often limited to quite a few scales (cells) : Lycœna argus, for instance, possesses a black spot on a particular part of the anterior wing consisting oí only ten scales, while the surrounding parts are blue. In this case we may therefore conclude that the black cells are represented in the germ-plasm by at least one determinant. The determination may possibly be carried out in still further detail in this instance, and each cell in the black spot may be determined from the germ onwards ; and possibly it is only the constant intermingling of two hereditary tendencies in sexual reproduction, and the consequent variability in the number of scales, which prevents us from recognising the fact. We can at any rate, however, find instances of the determination of single cells in other species of animals. For example, in many Crustaceans a number of sensory organs are situated on the anterior antennae : each of these corresponds to one cell. The number, position, and form of these ' olfactory ' setas is deter¬ mined exactly for each species. The Ostracod Cypris possesses only one olfactory seta on each antennule, while in the common fresh-water species of Gammarus, there are about twenty of these structures, each of which is separately attached to one