Volume 1

Essays upon heredity and kindred biological problems / by August Weismann ; edited by Edward B. Poulton, Selmar Schönland, and Arthur E. Shipley, authorized translation.

  • Shipley A. E. (Arthur Everett), Sir, 1861-1927.
Date:
1891-1892
Catalogue details

Licence: Public Domain Mark

Credit: Essays upon heredity and kindred biological problems / by August Weismann ; edited by Edward B. Poulton, Selmar Schönland, and Arthur E. Shipley, authorized translation. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    development is continued unchanged, and he therefore con- cluded that the fertilized egg contains within itself all the forces necessary for normal development. Finally, O. Hertwig' proved from observations on the eggs of sea-urchins, that at any rate in these animals, gravity has no directive influence upon segmentation, but that the position of the first nuclear spindle decides the direction which will be taken by the first divisional plane of segmentation. These observations were however still insufficient to prove that fertilization is nothing more than the fusion of nuclei ^ A further and more important step was taken when E, van Beneden ^ observed the process of fertilization in Ascaris mega- locephala. Like the investigations of Nussbaum* upon the same subject, published at a rather earlier date, van Beneden's observations did not altogether exclude the possibility of the participation of the body of the sperm-cell in the real process of fertilization; still the fact that the nuclei of the egg-cell and the sperm-cell do not coalesce irregularly, but that their loops are placed regularly opposite one another in pairs and thus form one new necleus (the first segmentation nucleus), dis- tinctly pointed to the conclusion that the nuclear substance is the sole bearer of hereditary tendencies—that in fact fertiHza- tion depends upon the coalescence of nuclei. Van Beneden himself did not indeed arrive at these conclusions : he was prepossessed with the idea that fertilization depends upon the union of two sexually differentiated nuclei, or rather half-nuclei —the male and female pronuclei. He considered that only in this way could a single complete nucleus be formed, a nucleus which must of course be hermaphrodite, and he believed that .the essential cause of further development lies in the fact that, 1 O. Hertwig,' Welchen Einfluss iibt die Schwerkraft,' etc. Jena, 1884. 2 [Our present knowledge of the development of vegetable ova (including the position of the parts of the embryo) is also in favour of the view that it is not influenced by external causes, such as gravitation and light. It takes place in a manner characteristic of the genus or species, and essentially depends on other causes which are fixed by heredity; see Heinricher, ' Beeinflusst das Licht die Organanlage am Farnembryo ?' in Mittheilungen aus dem Botanischen Institute zu Graz, II. Jena, 1888.—S. S.] „ ^, 3 E. van Beneden, ' Recherches sur la maturation de 1 oeuf, etc., i«»3- * M. Nussbaum, ' Ueber die Veranderung der Geschlechtsprodukte bis zur Eifurchung,' Arch. Mikr. Anat., 1884.