Module 4 Lesson 2 - 5

Lesson Summary

As implantation is being completed, the amniotic cavity develops within the amnion, a type of sac. From the amniotic cavity, the embryonic disk forms. This disk undergoes the process of gastrulation, which is the formation of germ tissue layers (endoderm, mesoderm, and ectoderm). The event of gastrulation leads to a new term for the developing offspring; it is now called a gastrula. During gastrulation, a series of changes occurs through which cells begin to differentiate.

Tremendous growth and organ development occur in the first trimester; 90% of all organs are formed by the end of the embryonic period. At the same time as the embryonic growth is happening, supporting structures are developing. The allantois, amnion, chorion, and yolk sac are formed to give support to the developing embryo and fetus. Some of these structures give rise to the placenta and umbilical cord. All the supporting structures help to nourish, protect, and remove wastes for the developing offspring during prenatal development. After the baby is born, all  supporting structures are expelled from the mother's body.