X

6 weeks

7 weeks

(b)

Figure 23.10

(a) In the fifth through the seventh weeks of gestation, the embryonic body and face develop a more humanlike appearance.

(b) A human embryo after about six weeks of development.

The umbilical cord also suspends the embryo in the amniotic cavity. The amniotic fluid provides a watery environment in which the embryo can grow freely without being compressed by surrounding tissues. The amniotic fluid also protects the embryo from being jarred by the movements of the woman's body.

In addition to the amnion and chorion, two other embryonic membranes form during development. They are the yolk sac and the allantois.

The yolk sac forms during the second week, and it is attached to the underside of the embryonic disk (see fig. 23.12). This structure forms blood cells in the early stages of development and gives rise to the cells that later become sex cells. The yolk sac also produces the stem cells of the bone marrow, which are precursors to blood cells, including those involved in the immune response. Portions of the yolk sac form the embryonic digestive tube as well. Part of the membrane derived from the yolk sac becomes incorporated into the umbilical cord, and the remainder lies in the cavity between the chorion and the amnion near the placenta.

The allantois (ah-lan'to-is) forms during the third week as a tube extending from the early yolk sac into the connecting stalk of the embryo. It, too, forms blood cells and gives rise to the umbilical arteries and vein (see figs. 23.12 and 23.15).

Eventually, the amniotic cavity becomes so enlarged that the membrane of the amnion contacts the thicker chorion around it. The two membranes fuse into an amniochorionic membrane (see fig. 23.16).

By the beginning of the eighth week, the embryo is usually 30 millimeters long and weighs less than 5 grams. Although its body is quite unfinished, it looks human (fig. 23.16).

The embryonic stage concludes at the end of the eighth week. It is the most critical period of development,

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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