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This is a 3µm longitudinal slice (#3) of the rat E10 egg cylinder.  The very top of the egg cylinder is the ectoplacental cone, the site of implantation to the uterine wall.  Beneath that, is the two-layered yolk sac, an outer layer of trophoblasts made up of extraembryonic endoderm.  The main function of these cells is to absorb nutrients from the yolk cavity, which is the empty space on the outside of the egg cylinder and the uterine tissue (not shown).  The inner layer of the yolk sac is extraembryonic mesoderm, which has characteristic swellings, blood islands—where blood cells are forming a circulatory system of the embryo.  The upper extraembryonic cavity is above the area where the embryo will develop. (An older embryological term for cavity is “coelom.”)  The double-layered amnion separates the embryonic and extraembryonic cavities.  The U-shaped embryo lies at the bottom of the egg cylinder; it is completely surrounded by the embryonic cavity and will fold up inside it as it grows.  The slice grazes the lateral edge of brain neuroepithelium that forms a hump just behind the prechordal plate—a region of mesoderm that induces head structures to develop.  The heart primordium lies above the prechordal plate.  Behind the neural folds is a more flattened area of neuroepithelium, the neural plate.  Just behind that is a layer of cuboidal cells that may be lateral surface ectoderm or possibly it is a lateral fold of the amnion.  In the embryonic part of the egg cylinder, the outermost layer of spiky cuboidal to squamous cells is the embryonic endoderm.  The embryonic mesoderm has several layers of rounded to stellate-shaped cells, arranged in varying densities beneath the neuroepithelium.  (A=anterior, P=posterior)