Formation of the Germ
After implantation, a new cavity, called the amniotic (am-nē-ot′ ik) cavity, forms inside the inner cell mass and causes the part of theinner cell mass nearest the blastocele to separate as a flat disk of tissue called the embryonic disk (figure 20.5). The amniotic cavity is bounded by a membrane called the amnion and is filled with amniotic fluid. The embryo will grow in the amniotic cavity,where the amniotic fluid forms a protective cushion. The embryonic disk at first is composed of two layers of cells: an epiblast adja-cent to the amniotic cavity and a hypoblast on the side of the disk opposite the amniotic cavity. A third cavity, the yolk sac, forms inside the blastocele from the hypoblast.
At about 14 days after fertilization, the embryonic disk has become a slightly elongated, oval structure. Some of the epiblast cells migrate toward the center of the disk, forming a thickened
At about 14 days after fertilization, the embryonic disk has become a slightly elongated, oval structure. Some of the epiblast cells migrate toward the center of the disk, forming a thickenedline called the primitive streak (figure 20.6). The formation of the primitive streak establishes the central axis of the embryo. Some of the epiblast cells migrate through the primitive streak. Of these migrating cells, some displace the hypoblast to form the endoderm (en′ dō-derm; inside layer) while others emerge between the epi-blast and endoderm as a new germ layer, called the mesoderm (mez′ ō-derm; middle layer). Those epiblast cells that do not migrate form the ectoderm (ek′ tō-derm; outside layer). This pro-cess of cell migration and the formation of three distinct germ layersis called gastrulation. The embryo is now three-layered, having ectoderm, mesoderm, and endoderm (figure 20.6). All the tissues of an adult can be traced to these three germ layers (table 20.1).
From about day 14 until about day 35, the embryo is at maxi-mum risk from environmental toxins and drugs that can cause birth defects. The causes of birth defects are a major unsolved issue in biology at present. However, it appears that oxidative damage to key molecules in certain developing cells and/or changes in the DNA function within those cells may be involved.
A specialized group of cells at the cephalic (towards the head) end of the primitive streak moves from one end of the primitive streak to the other and, in some yet unknown way, organizes the embryo. A cordlike structure called the notochord (n̄o ′ t ̄o -k̄o rd) is formed by these cells as they move down the primitive streak. The notochord marks the central axis of the developing embryo (figure 20.6).