This is a 3µm transverse slice (#11) of the rat E10 egg cylinder. The slice cuts the most anterior part of the embryo on the left and is still above the level of the embryo on the right. Surrounding uterine tissue is not in the picture. All of the structures seen in slices #1 and 2 are in this slice as well as many new structures.
The amnion is a double-walled membrane of squamous (flat) cells. The layer closest to the embryo is ectoderm; the layer next to the extraembryonic cavity is mesoderm.
The extraembryonic (amniotic) cavity is the space surrounded by the amnion. This is a fluid-filled cavity in which the embryo curls up and grows.
The prechordal plate is a layer of cuboidal cells that are axial (midline) mesoderm cells. This plate induces the development of head structures in the embryonic body axis.
The heart primordium is composed of a cardiogenic plate (forerunner of the heart itself) and a pericardial cavity (the space that will contain the heart).
The embryonic endoderm is the outermost layer of squamous to cuboidal cells that will eventually invaginate and form the lining of the mouth.