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This is a 3µm longitudinal slice (#50) of the rat E10 egg cylinder that cuts across a definite midline area in upper parts of the spinal cord (or possibly lower medulla).  The midline area has a floor plate (an early maturing structure) in the neuroepithelium and a notochordal plate below.  The neuroepithelium on the right side of the brain is forming a hump in the forebrain and midbrain and a concavity in the center of the hindbrain—most likely the future pontine flexure.  The spinal cord extends farther up the posterior part of the U-shaped embryo and is continuous with the primitive streak.  The mesoderm is diffuse beneath the anterior brain and compact beneath the posterior hindbrain—a forerunner of the future somites.  Another new feature is the allantois in the posterior extraembryonic cavity.  This structure accumulates metabolic waste products and exchanges gases with the maternal blood circulation.