Volume 2

A treatise on comparative embryology / by Francis M. Balfour.

  • Francis Maitland Balfour
Date:
1880-1881
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on comparative embryology / by Francis M. Balfour. Source: Wellcome Collection.

    72/700 (page 56)
    The segmentation, which is preceded by active movements of the germinal disc, is meroblastic. There is nothing very special to note with reference to its general features, but while in large ova like those of the Salmon the first furrows only penetrate for a certain depth through the germinal disc, in small ova like those of the Herring, they extend through the whole thickness of the disc. During the segmentation a great increase in the bulk of the blas- toderm takes place. In hardened specimens a small cavity amongst the segmentation spheres may be present at any early stage; but it is probably an arti- ficial product, and in any case has nothing to do with the true segmentation cavity, which does not appear till near the close of segmentation. The peripheral layer of granular matter, continuous with the germinal disc, does not undergo division, but it becomes during the segmentation specially thickened and then spreads itself under the edge of the blastoderm ; and, while remaining thicker in this region, gradually grows inwards so as to form a continuous sub- blastodermic layer. In this layer nuclei appear, which are equivalent to those in the Elasmobranch ovum. A considerable number of these nuclei often become visible simultaneously (van Beneden, No. 60) and they are usually believed to arise spontaneously, though this is still doubtful \ Around these nuclei portions of protoplasm are segmented off, and cells are thus formed, which enter the blasto- derm, and have nearly the same destination as the homologous cells of the Elasmobranch ovum. During the later stages of segmentation one end of the l)lastoderm becomes thickened and forms the embryonic swelling ; and a cavity appears between the blastoderm and the yolk which is excentrically situated near the non-embryonic part of the blastoderm. This cavity is the true segmentation cavity. Both the cavity and the embryonic swelling are seen in section in fig. 31 A and B. In Leuciscns rutilus Banibeke describes a cavity as appearing in the middle of the blastoderm daring the later stages of segmentation. From his figures it might be supposed that this cavity was equivalent to the segmentation cavity of Elasmobranchs in its earliest condition, but Bam- beke states that it disap])eais and that it has no connection with the true segmentation cavity. Bambeke and other investigators have failed to recognize the homology of the segmentation cavity in Teleostei with that in Elasmobranchii, Amphibia, etc. With the appearance of the segmentation cavity the portion of the blastoderm which forms its roof becomes thinned out, so that the whole blastoderm consists of (1) a thickened edge especially prominent at one point where it forms the embryonic swelling, and (2) a thinner central portion. The changes which now take place result in the differentiation of the embryonic layers, and in the rapid 1 Vide Vol. I. p. 89.