Читать книгу Lifespan Development - Tara L. Kuther - Страница 151

During the germinal period, also known as the period of the zygote, the newly created zygote begins cell division as it travels down the fallopian tube, where fertilization took place, toward the uterus. About 30 hours after conception, the zygote then splits down the middle, forming two identical cells (Webster et al., 2018). This process is called cleavage, and it continues at a rapid pace. As shown in Figure 3.2, the two cells each split to form four cells, then eight, and so on. Each of the resulting cells is identical until about the third set of cell divisions. This process of cell division continues rapidly. Any of these cells may become a person (and sometimes do, in the case of monozygotic or identical twins).

This ball of cells, known as a morula, is formed at about three days after conception. Each of these cells is identical. Differentiation has not yet begun.

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Cell differentiation begins roughly 72 hours after fertilization when the organism consists of about 16 to 32 cells. Differentiation means that the cells begin to specialize and are no longer identical. By 4 days, the organism consists of about 60 to 70 cells formed into a hollow ball called a blastocyst, a fluid-filled sphere with cells forming a protective circle around an inner cluster of cells from which the embryo will develop.

Development proceeds very quickly during the embryonic period. Note the dramatic changes from the fifth week (left) to the seventh week (right) of prenatal development.

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Description

Figure 3.2 Germinal Period

Source: Levine and Munsch (2010, p. 102).

Implantation, in which the blastocyst burrows into the wall of the uterus, begins at about day 6 and is complete by about day 11 (K. L. Moore & Persaud, 2016). By the end of the second week, when fully implanted into the uterine wall, the outer layer of the blastocyst begins to develop into part of the placenta, the principal organ of exchange between the mother and developing organism. The placenta will enable the exchange of nutrients, oxygen, and wastes via the umbilical cord. Also, during this stage, the developing organism is encased in amniotic fluid, providing temperature regulation, cushioning, and protection from shocks.