Text-book of the embryology of invertebrates / by E. Korschelt, K. Heider.

  • Korschelt, E. (Eugen), 1858-1946. Lehrbuch der vergleichenden Entwicklungsgeschichte der wirbellosen Thiere. English
Date:
1895-1900
Catalogue details

Licence: In copyright

Credit: Text-book of the embryology of invertebrates / by E. Korschelt, K. Heider. Source: Wellcome Collection.

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    Separation of the cells there soon arises a central cavity, the cleavage cavity or Von Baers cavity (blastoccele), whicti con- tinnally increases during the succeeding cell divisions, while the blastomeres arrange themselves about this cavity in a single-layer epithelium (blastoderm). The stage of develop- ment thus reached is known as the blastula or blastosphere. In the one-layer blastula an arrangement of the parts of the egg about the chief axis is also clearly recognizable. The ■ cells in the vicinity of the animal pole are, as a rule, smaller and not so rieh in nutritive yolk particles; whereas the cells of the vegetative portion are larger and richer in yolk, and, in consequence of the impeding influence offered by the nutritive yolk, divide more slowlv.1 The wall of the single- layer blastosphere represents the first of the primitive organs of the Metazoan body. In the simplest cases a gastrula-stage is developed: out of the blastula-stage by the cell-layer of the vegetative half becoming flattened and gradually depressed, so that there arises an ever-deepening invagination at the vegetative pole. In this way the cleavage cavity (primitive body cavity) becomes gradually reduced, and often is preserved only as a narrow cleft between the two layers of the body-wall produced by the metamorphosis already described. The gastrula-stage has substantially the form of a sac. It encloses a cavity which has arisen by invagination, called the archenteric cavity, and opens to the exterior in the region of the vegetative pole by means of the primitive mouth or prostoma {blastopore). The wall at this stage consists of two cell-layers : an outer, the ectoderm, which is derived from the cells of the animal portion of the blastosphere, and an inner, the entoderm, which consists of the cells of the formei vegetative half, and which has reached the inside of the embryo by the process of invagination. In the region of the blastopore the ectodermal and entodermal layers be- come continuous with each otlier. Ectoderm and entoderm 1 There are reasons for thinking that the rate of cleavage is not wliolly dependent on the proportion of nutritive yolk in the blastomere. (See Ivofoid, C. A., “On Some Laws of Cleavage in Limax.” Proc. Ämer. Acacl. Arts and Sciences, vol. xxix., p. 180, 1894) [Translators] .