The early embryos at the embryonal cylinder stage in rats were studied by scanning electron microscopy, employing the frozen resin-cracking method. An embryo at 15:00 o’clock on day 7 of pregnancy (L6-15) was exposed in the frozen resin-cracked surface. It was located at the antimesometrial side of the uterus and constructed of the embryonic node and trophoblasts, enclosing the blastocele. The embryonic node consisted of the primary ectodermal cell mass and the primary entodermal cells, surrounding the former. The ectodermal cells, forming a solid mass, are polygonal in shape and contain each a comparatively very large nucleus having a significant nucleolus. The entodermal cells are simple columnar or cuboidal in shape, contain numerous vacuoles of various sizes, in which threadlike structures forming a network are seen, and surround closely the ectodermal cell mass; they continue at the ectoplacental cone to the distal entodermal cells, which are very flattened, simple squamous in shape and line the inner surface of Reichert’s membrane. In this specimen, the endometrial luminal epithelium has disappeared completely; the narrow blastocele is observed, but the primary amniotic cavity is not.

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