Letter | Published:

Mouth and Blastopore

Naturevolume 187page1132 (1960) | Download Citation



BY the Haeckelian view of early development, gastrulation by invagination reflects the evolutionary history of primitive Metazoa. It is a corollary of this theory that gastrulation by ingression is a derived form of emboly. Nevertheless ingression, the method of endoderm formation among the lower cnidarians, is probably the more primitive mode of gastrulation, as observed by Metschnikoff1,2, Hyman3,4 and others. The diploblastic level of organization probably arose in early Metazoa by a process resembling ingression, or some other type of delamination, giving rise to solid planula-like forms. It is therefore probable that invagination is merely a developmental expedient, which has arisen secondarily and independently in the more advanced Cnidaria and other metazoan groups.

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  1. 1

    Metschnikoff, E., Z. Wiss. Zool., 36 (1882).

  2. 2

    Metschnikoff, E., “Embryologische Studien an Medusen” (1887).

  3. 3

    Hyman, L. H., “The Invertebrates”. 1. “Protozoa through Ctenophora” (New York and London, 1940).

  4. 4

    Hyman, L. H., Biol. Symposia, 8, 1 (1942).

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  1. London School of Hygiene and Tropical Medicine, Keppel Street, London, W.C.1



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