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Drawing made from a graphic reconstruction and a model of specimen no. 7801, representing the right half of the embryo.

The germ disk is composed of closely packed cells, and it is thick throughout. The primordium of the primitive streak occurs in only a few sections at the caudal end of the embryo. A dense mass of undifferentiated cells, caudad to the streak and fused with the end of the germ disk and the amniotic ectoderm, is evidently contributing cells to the mesoderm of the body stalk. Most of the latter tissue, however, is presumably a derivative of cytotrophoblast. The extra-embryonic mesoderm is a spongy tissue composed of small variously shaped fluid-filled compartments. Parts of the walls of these spaces are very thin. In representing them plastically, however, the thickness is somewhat exaggerated. The gut endoderm is a thick plate, but the constituent cells presumably have lost a considerable amount of lipoid material, with the result that the layer has a foamy appearance in sections. (see pl. I, fig. 4).

Fig. 13. Heuser et al., 1945.

Keywords: amnion, amniotic duct, amniotic ectoderm, chorionic villi, cytotrophoblast, extra-embryonic mesoderm, gut endoderm, lipoid material, mesoderm of the body stalk, primitive streak, primordium of the primitive streak, secondary umbilical vesicle

Source: The Virtual Human Embryo.