Fetal Development

1. By 7 days after fertilization, the embryo has over 10 cells arranged around a central cavity. This is the embryonic stage known as the blastocyst. Protruding into one end of the blastocyst cavity is a cluster of cells called the inner cell mass, which will subsequently develop into the embryo proper and some of the extraembyonic membranes. The outer epithelium surrounding the cavity is the trophoblast, which, along with mesodermal tissue, will form the fetal portion of the placenta. The embryo reaches the uterus by the blastocyst stage and soon begins to implant in the uterine lining (endometrium).
2. The trophoblast initiates implantation by secreting enzymes that enable the blastocyst to penetrate the endometrium. Bathed in blood spilled from eroded capillaries in the endometrium, the trophoblast thickens and extends fingerlike projections into the surrounding maternal tissue. (The placenta will later form from this proliferated trophoblast and the region it invades. Around the time of implantation, the inner cell mass of the blastocyst forms a falt disc with an upper layer cells, the epiblast, and the lower layer the hypoblast.
3. Now the extraembyonic membranes begin to develop . the trophoblast is iving rise to the corion and continues to expend into the endometrium. The epiblast has begun to form the amnion, surrounding a fluid-filled cavity. Mesodermal cells that will become part of the lacenta are also derived from the epiblast.
4. Grastulation occurs by the inward movement of cells from the epiblast through a primitive streak to form mesoderm and endoderm, just as it does in the chick. We now have a three-layered embryo surrounded by a proliferating extraembryonic mesoderm. Four extraembryonic membranes have formed. The chorion, which develops from the trophoblast, completely surrounds the embryo and the other extraembryonic membranes. The amnion begins as a dome above the proliferating epiblast and will eventually enclose the embryo in a fluid-filled amniotic cavity.below the developing embryo proper, the yolk sac encloses another fluid-filled cavity. The yolk sac membrane is a site of early formation of blood cells, which later migrate into the embryo proper. The fourth extraembryonic membrane, the allantois, develops as an outpocketing of the embryo’s rudimentary gut, as it does in the chick. The allantois is incorporated into the umbilical cord, where it forms blood vessels that transport oxygen and nutrients from the placenta to the embryo and rid the embryo of carbon dioxide and nitrogenous waste.