This is a 3µm transverse slice (#1) of the rat E10 egg cylinder; the slice is above the level of the embryo.  Surrounding uterine tissue is not in the picture.  The egg cylinder wall 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 rounded rectangular cavity and the uterine tissue (not shown).  The inner layer is extraembryonic mesoderm and has characteristic swellings, blood islands, where hematopoetic tissue is forming and will eventually participate in the circulatory system of the embryo.  The large central cavity is above the area where the embryo will develop and is called the extraembryonic cavity. (An older embryological term for cavity is “coelom.”)  The only structure within the extraembryonic cavity is the allantois, a membranous sac that acts to absorb oxygen, release carbon dioxide, and accumulate metabolic waste away from the embryo.  Eventually, the allantois and the yolk sac will become part of the umbilical cord to the placenta where maternal and embryonic tissues interact to provide nourishment and remove waste materials. (A=anterior, P=posterior)