Human embryology / by Charles Sedgwick Minot.

  • Charles Sedgwick Minot
Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Human embryology / by Charles Sedgwick Minot. Source: Wellcome Collection.

    64/854 page 32
    The embryo differs greatly from the three months' foetus. Be- ginning with the envelopes we notice that the chorion is beset with well-developed villi over its entire surface, but the villi over the pla- cental area are larger than those over the parts adjacent to the de- cidua reflexa. The amnion does not lie close to the chorion, but close around the embryo, leaving a wide space between the two mem- branes, which space, as we have seen, is subsequently obliterated by the expansion of the amnion. The embryo itself is very small and not human in appearance, and its organs are only partially differen- tiated. The umbilical cord is very short; the amnion springs from it at a short distance from the embryo. The yolk-stalk leaves the cord just beyond the amnion, is comparatively short, and ends in the pear-shaped yolk-sack, which is about the same size as at three months. Beyond the point where the amnion and yolk-stalk part from it, the umbilical cord continues a short distance with its blood- vessels, which ramify over the entire chorion and penetrate all the villi thereof. To produce the relations found at three months the blood-vessels and villi of the chorion must abort except over the pla- cental area; the umbilical cord must elongate greatly; the amnion must expand until it touches the . chorion, and the foetus must grow \ and change. / \ We must now trace back the his- I . tory of the embryo still farther, in ^ ^ order to understand the relation of ^Y-iQ embryo to the embryonic mem- . ' # / \ branes and appendages. I \ Ovum of Three Weeks.*—Nor- mal human ova of this age very ■ rarely indeed reach the embryolo- . gists, but a few have been described. ? The chorion forms a closed vesicle beset on all sides with crowded, clumsily-branching villi; the vesicle measures about 3 cm. in diameter; the villi are about 3 mm. long, and as yet show no regional inequality in their development. If the vesicle is opened the embryo is found within rolled up, the back being convex; it measures in its natural attitude about 44 mm. The head is bent toward the right; the caudal extremity to- ward the left; the head and tail are almost in contact, so that it is diffi- cult to observe the insertion of the umbilical cord. With care this may be done, and it will then be seen that the amnion arises from the embryo, and is, in fact, a prolongation of the body-wall; the amnion itself is extremely thin and lies close about the embryo. The umbilical * For figures see Chapter XIII. Fig. 16.—Human embryo, 4.2 mm. long (His' Lr). After W. His. Explanation in text.
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