This is a 3µm transverse slice (#55) of the rat E10 egg cylinder. Surrounding uterine tissue is not in the picture. The slice is near the bottom of the U-shaped embryo and cuts through a thick ring of neuroepithelium that falsely gives the impression of a neural tube. Notice the greater narrowing of the embryonic cavity from slice #50. The notochordal plate is now present beneath the neural groove both anteriorly and posteriorly. The mesoderm is very thick and compact on lateral sides of the neuroepithelium.