The elements of embryology. / By M. Foster ... and Francis M. Balfour.

  • Michael Foster
Date:
1874
Catalogue details

Licence: Public Domain Mark

Credit: The elements of embryology. / By M. Foster ... and Francis M. Balfour. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

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    thicker than in the rest of the pellucid area. It is apparently this median i thinning of the mesoblast which gives rise to the linear transparency seen in ] specimens viewed with transmitted light. The hypoblast, it may be remarked, is generally curved dowTiwards beneath the primitive streak and groove, though ! not to the same extent as the epiblast. Thus the whole blastoderm is some- what curved in this region. Immediately beneath the groove a kind of fusion takes place between the epiblast and mesoblast, though on close examination [ the line of junction between them can generally be made out. This apparent fusion His (Ueber die Erste Anlage des Wirheltheirleibs) regarded as an event of great importance, and gave the name of axis-cord to the part in which it occurs. In fresh specimens a narrow (opaque) streak can be seen running down the centre of the groove; but it is not represented by any structure which can be seen in sections. The chief events then which occur during the first twelve hours of incubation are the establishment of the three layers of the blastoderm, and the appearance of the embryonic shield, of the primitive streak and of the primitive groove, 5. Fo'om the 12th to the 20th hour.—During this period | the pellucid area rapidly increases in size, and from beiug j oval becomes pear-shaped. The primitive groove grows even more rapidly than the pellucid area; so that by the 16th hour it is not only absolutely, but also relatively to the pellucid | area, longer than it was at the 12th hour. The interval ; between its end and the circumference of the pellucid area .} continues to be greater in front than behind. At about the 16th hour, or a little Ijiter, a thickening of I the mesoblast takes place in front of the primitive_ groove, : giving rise to an opaque streak ending abruptly in front against a semicircular fold, which appears at this time near ; the anterior extremity of the pellucid area (Fig. 11), and ) is known as the head-fold. In fresh specimens this streak | looks like a continuation from the anterior extremity of the i primitive groove; but in hardened specimens it is easy to see that the connection is only an apparent one. Along the new streak a groove (Fig. 11, m. c.) is very soon ■ formed, which, narrow in front, but widening very much behind, embraces between its diverging walls the anterior extremity of the primitive groove. This new groove, by the conversion of which into a tube the medullary canal will be formed, is known as the medullary groove. On each side of it the mesoblast is thickened, and the surface of the blastoderm raised up in the form of two longi- tudinal folds, known as the lamince dorsales, or the medullary folds (Fig. li, Immediately beneath the bottom of the