outer, termed the syncytium or synctiotrophoblast, so named because it consists of a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells; and an inner layer, the cytotrophoblast or layer of Langhans, in which the cell outlines are defined. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the chorion and play an important part in the development of the placenta. On the deep surface of the inner cell-mass a layer of flattened cells, the entoderm, is differentiated and quickly assumes the form of a small sac, the yolk-sac. Spaces appear between the remaining cells of the mass (Fig. 11), and by the enlargement and coalescence of these spaces a cavity, termed the amniotic cavity (Fig. 12), is gradually developed. The floor of this cavity is formed by the embryonic disk composed of a layer of prismatic cells, the embryonic ectoderm, derived from the inner cell-mass and lying in apposition with the entoderm.
The Primitive Streak; Formation of the Mesoderm.—The embryonic disk becomes oval and then pear-shaped, the wider end being directed forward.
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Fig. 13. — Surface view of embryo of a rabbit. (After Kölliker.) arg. Embryonic disk. pr. Primitive streak.
Near the narrow, posterior end an opaque streak, the primitive streak (Figs. 13 and 14), makes its appearance and extends along the middle of the disk for about one-half of its length; at the anterior end of the streak there is a knob-like thickening termed Hensen's knot. A shallow groove, the primitive groove, appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the blastophore, with the yolk-sac. The primitive streak is produced by a thickening of the axial part of the ectoderm, the cells of which multiply, grow downward, and blend with those of the subjacent entoderm (Fig. 15). From the sides of the primitive streak a third layer of cells, the mesoderm, extends lateralward between the ectoderm and entoderm; the caudal end of the primitive streak forms the cloacal membrane.
The extension of the mesoderm takes place throughout the whole of the embryonic and extra-embryonic areas of the ovum, except in certain regions. One of these is seen immediately in front of the neural tube. Here the mesoderm extends forward in the form of two crescentic masses, which meet in the middle line so as to enclose behind them an area which is devoid of mesoderm. Over this area the ectoderm and entoderm come into direct contact with each other and constitute a thin membrane, the buccopharyngeal membrane, which forms a septum between the primitive mouth and pharynx. In front of the buccopharyngeal area, where the lateral crescents of mesoderm fuse in the middle line, the pericardium is afterward developed, and this region is therefore designated the pericardial area. A second region where the mesoderm is absent, at least for a time, is that immediately in front of the pericardial area. This is termed the proamniotic area, and is the region where the proamnion is developed; in man, however, a proamnion is apparently never formed. A third region is at the hind end of the embryo where the ectoderm and entoderm come into apposition and form the cloacal membrane. The blastoderm now consists of three layers, named from without inward: ectoderm, mesoderm, and entoderm; each has distinctive characteristics and gives rise to certain tissues of the body.[1]
Ectoderm.—The ectoderm consists of columnar cells, which are, however, somewhat flattened or cubical toward the margin of the embryonic disk. It forms the whole of the nervous system, the epidermis of the skin, the lining cells of the sebaceous,
- ↑ The mode of formation of the germ layers in the human ovum has not yet been observed; in the youngest known human ovum (viz., that described by Bryce and Teacher), all three layers are already present and the mesoderm is split into its two layers. The extra-embryonic celom is of considerable size, and scattered mesodermal strands are seen stretching between the mesoderm of the yolk-sac and that of the chorion.
