This is a 3µm transverse slice (#45) of the rat E10 egg cylinder. Surrounding uterine tissue is not in the picture. The slice is now deeper in the U-shaped embryo and cuts through a thick ring of neuroepithelium that falsely gives the impression of a neural tube. The embryonic cavity is narrowing on each side of a midline that runs through the center of the neural groove (above the notochordal plate) and the center of the neural plate (above the primitive streak). The compact nature of the mesoderm is most obvious at a middle anterior-posterior level; these will become the first somites that will develop within the next few hours.