This is a 3µm transverse slice (#40) of the rat E10 egg cylinder.  Surrounding uterine tissue is not in the picture.  The slice is deep enough in the U-shaped embryo to cut through a complete ring of neuroepithelium, most likely in the region of the hindbrain (with a neural groove anteriorly) and the upper spinal cord (with a neural plate posteriorly).  The ring of neuroepithelium appears to be a tube, but it is still open.  Notice the mesoderm is compact in the posterior half of the embryo, less so anteriorly.