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This is the yelk-sac, or " umbilicalvesicle," the origin of which we have considered previously.
2
The yelk-sac is reduced to a small pear-shaped umbilicalvesicle; its thin pedicle, the long vitelline duct, is enclosed in the umbilical cord.
3
These vessels naturally atrophy with the degeneration of the umbilicalvesicle, and the vitelline circulation is replaced by a second, that of the allantois.
4
H The vertebrae (w) have grown round the medullary tube (mr) and chorda; the body-cavity is closed, and the umbilicalvesicle has disappeared.
5
The very wide opening, through which the gastric cavity at first communicates with the umbilicalvesicle, becomes narrower and narrower, and at last disappears altogether.
6
D The skin-fibre-layer (hf) and gut-fibre-layer (df) divide at the periphery; the gut (d) begins to separate from the yelk-sac or umbilicalvesicle (nb).
7
AC amniotic cavity, UV yelk-sac or umbilicalvesicle, ALC allantois, al pericoelom or serocoelom (inter-amniotic cavity), sz serolemma (or serous membrane), pc prochorion (with villi).)
8
Later still, the remainder of the food-yelk only forms a small round sac, the yelk-sac or umbilicalvesicle (Figure 1.105 nb).