File:Mammals The separation of the discoidal germ from the yolk-sac, seen in section.jpg

Fig. 100.--The separation of the discoidal mammalian germ from the yelk-sac, seen in section (diagrammatic). A. The germ-disc (h, hf) lies extended on one side of the intestinalgerm-vesicle (4b). B. In the centre of the germ-disc the medullary furrow (mr), and under that the notochord (ch) appear. C. The intestinal-fibrous layer (df) has growm round the intestinal-glandular layer (dd). D. Skin-fibrous layer (hf) and intestinal-fibrous layer (df) part round the circumference of the germ-disc ; the intestine (d) begins to separate itself from the yelk-sac or navel-vesicle (nb). E. The intestinal tube (mr) is closed; the body-cavity (c) begins to form. F. The primitive vertebrea (w) appear; the intestine (archenteron) (d) is almost completely closed. G. The primitive vertebrae (w) begin to grow round the medullary tube (mr) and the notochord (ch); the intestine (d) is separated from the navel-vesicle (nb). H. The vertebrae (w) have enclosed the medullary tube (mr) and the notochord (ch); the body-cavity (c) is closed ; the navel-vesicle has disappeared. The amnion and serous membranes are omitted.