implantation of the blastocyst usually occurs 6 to 8 days after fertilization by the end of day 8 the blastocyst has borrowed into the endometrium of the uterus at this time it is composed of two main components the outer cell Mass the Tropa blast and the Inner Cell Mass the embrya blast as the Tropa blast makes contact with the endometrium it differentiates into two layers an inner cytotrophoblast and an outer sinot Tropa blast the embryoblast differentiates into a bilaminar embryonic dis composed of two cell layers the hypoblast and the epiblast soon after the embryonic disc has
formed a cavity begins to appear between the epiblast and the cytotrophoblast known as the amniotic cavity cells originating from the hypoblast begin to migrate forming a thin membrane which covers the inner surface of the cyot Tropa blast this is called the exosomic membrane the exosomic membrane and cells of the hypoblast together form the walls of the primitive yoke sack by Day N the blastocyst is completely embedded in the uterus wall at this stage of development the growth of the ciot Tropa blast and cytotrophoblast is much quicker than the bamer embryonic disc small holes called launi
begin to form in the cciio Tropa blast as it continues to expand by day 12 the lacuni stop growing and fuse to form large interconnecting spaces called Lacuna networks capillaries in the endometrium surrounding the developing embryo dilate forming maternal sinusoids as the sinot Tropa blast continues to expand enzymes begin to erode the lining of the sinusoids and uterine glands allowing maternal blood and uterine secretions to flow into the luner neck networks establishing a uteroplacental circulation the blood and uterine secretions only come into close proximity to the embryo allowing the exchange of gases and metabolites around
the same time a new population of cells appear between the inner surface of the cyot Tropa blast and the outer surface of the Primitive yolk Sac known as the extra embryonic mesoderm large cavities begin to appear in the extraembryonic mesoderm these gradually fuse to form one single cavity called the chonic cavity around 13 days after fertilization a large portion of the exosomic cavity is pinched off forming a smaller cavity the secondary yolk sack by the end of the second week of development the chonic cavity enlarges and the bilaminar embryonic disc is joined to the Tropa
Blast by a band of extraembryonic mesoderm called the connecting stock the future umbilical cord
