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.

    167/522 (page 137)
    REGENERATION. I37 from his observations ; Driesch,* however, has made certain deductions from a series of similar experiments on the eggs of Sea-urchins. By continued shaking, Driesch effected a mechani¬ cal separation of the two first segmentation-cells, and observed that at first each of them continued to undergo further segmenta¬ tion just as would occur in the entire egg, but that later on the resulting ^^/«/-blastula became completed to form an entire one. In some of these hemi-blastulae development proceeded still further, the invagination taking place to form the primary digestive cavity of the gastrula, so thai eventually a rudimentary pluteus-larva^—which, though small, was in other respects nor¬ mal—could be recognised. Driesch sums up his results in the following words :—'These experiments therefore show that under certain circumstances each of the two first segmentation-cells of Echinus micro-tuber- culatus can give rise to a larva of the normal form, which is entire as regards its shape ; and that a partial formation, and not a semi-formation, occurs in this case.' The author con¬ cludes that his results ' fundamentally disprove the existence of special regions in the germ which give rise to special organs,' and adopts the following view stated by Hallezt :—' II n'est pas dès lors permis de croire que chaque sphère de segmentation doit occuper une place et jouer un rôle, qui lui sont assignés à l'avance.' Although I am far from wishing to assert that we are at present in a position to give a perfectly reliable and detailed explanation of the extremely interesting and important results of the experiments just described, I nevertheless cannot help thinking that they do not in the least necessitate the giving up of the view which entails a predestination of the individual segmentation-cells, and, in fact, of cells in general. Other than experimental methods may lead us to fundamental views, and an experiment may not always be the safest guide, although it may at first appear perfectly conclusive. Even Driesch himself doubts whether the above-mentioned experiments made by Roux are really conclusive, though, in my opinion, he is wrong in doing so ; he asks, in fact, whether the uninjured segmenta¬ tion-sphere of the frog would not behave exactly in the same * H. Driesch, ' Entwicklungsmechanische Studien, Zeitschrift f. wiss. Zoologie, Bd. ^з, 1891. t Hallez, ' Recherches sur rembryologie des Nématodes,' Paris, 1885.