This is a 3µm transverse slice (#20) of the rat E10 egg cylinder. The slice cuts the anterior part of the embryo on the left and its most posterior part on the right. Surrounding uterine tissue is not in the picture. All of the structures, except the allantois, seen in the previous slices are in this slice but in new configurations.
The amnion is cut in two places that define anterior and posterior embryonic cavities. The extraembryonic cavity is shrinking into two lateral wedge-shaped parts.
The neuroepithelium of the brain is two large mounds of cells (the neural folds) on either side of a neural groove in the midline. The head mesoderm is a loose array of large cells beneath the neural fold arches. The notochordal plate is the layer of mesendodermal cells just beneath the neural groove and is responsible for the establishment of the midline ventral part of the nervous system and the axial skeleton. The anterior embryonic endoderm is continuous with the notochordal plate. The neuroepithelium of the spinal cord is a slightly curved neural plate. In the primitive streak, cells are moving into the mesoderm beneath the midline area of the neural plate. On either side of the primitive streak area, the dense trunk mesoderm is accumulating above the posterior embryonic endoderm, which will eventually invaginate and become part of the hindgut.