Lesson 2 — Embryonic Development



Self-Check

  1. What is the amniotic cavity, and where does it form?

  2. Name the three layers of the embryonic disk.

  3. What is the process that results in the formation of the three germ layers?

  4. What is morphogenesis?

  5. Explain the differentiation of the three germ layers.

  6. What is neurulation?

  7. What events occur during the third week of embryonic development?

  8. What events occur during the fourth week of embryonic development?

  9. What events occur between the fourth and eight weeks of embryonic development?

  10. When does an embryo become a fetus?

  11. Identify the extra embryonic membranes.

  12. From where do the placenta and umbilical cord develop?

  13. What is the role of the placenta?

  14. What is the role of the umbilical cord?


  1. The amniotic cavity is a space that forms between the inner cell mass and the trophoblast. This space fills with fluid, and this is where the embryo will develop, within the sac known as the amnion.

  2. The embryonic disk is composed of three layers: ectoderm, mesoderm, and endoderm.

  3. The development of the primary germ layers is called gastrulation. The developing embryo at this point is called the gastrula.

  4. Morphogenesis, which begins with gastrulation, is a series of events that form the distinct structures of the developing embryo. It depends on the ability of embryonic cells to change into various types of cells.

  5. The development of the primary germ layers produces three distinct layers of cells: endoderm, mesoderm, or ectoderm. The cells in the primary germ layers are the source of all organs and tissues of the body. The process of the development of these cells into particular shapes able to perform particular functions is called differentiation.

  6. Neurulation is the development of the neural tube (located immediately above the notochord), which develops into the brain and the spinal cord. Cells along the surface of the notochord begin to thicken. As the cells thicken, folds develop along each side of a groove along this surface. As the folds fuse, they become the neural tube.

  7. During the third week, a thickened band of mesoderm cells (a primary germ layer) develops along the back of the embryonic disk. These cells lie along what will become the baby's back and come together to form the notochord (the basic framework of the skeleton). In addition, a small reddish bulge that contains the heart forms; the heart begins to beat on the 18th day.

  8. During the fourth week, the blood cells are forming and beginning to fill the developing blood vessels; the lungs and kidneys are taking shape; small buds that will become limbs form; a distinct head, with early evidence of eyes, ears, and a nose, is visible.

  9. Students will list any four of the following: In the fifth week, the lidless eyes are open, and the cells of the brain are differentiating very quickly. At the sixth week, the brain is continuing to develop; the limbs are lengthening and can flex; the gonads produce hormones to influence the development of the external genitalia. In the seventh and eighth weeks, the organs are fully formed; the nervous system is coordinating the body activity; a skeleton of cartilage has formed; eyes are well developed, but not open; nostrils are developed but are plugged with mucus; external genitalia are developing, but are undifferentiated at this point. Upon completion of the eighth week, approximately 90% of the organs and structures are established, and they continue to enlarge and mature.

  10. The embryo is identified as a fetus ("offspring") after the eighth week of pregnancy when the organs and structures are established.

  11. The extra-embryonic membranes are the allantois, the amnion, the chorion, and the yolk sac.

  12. One portion of the placenta develops from the chorion. The allantois is the foundation of the umbilical cord.

  13. The placenta has nutritional, excretory, respiratory, endocrine, and immune functions.

    •  Nutritional function: transports nutrients such as glucose, amino acids, and fatty acids; stores nutrients (protein, fats, carbohydrates, minerals) to be released later in the pregnancy

    • Excretory function: transports wastes such as urea, ammonia, and creatinine from the fetal blood to the mother's blood

    • Respiratory function: transports oxygen from mother to fetus, and carbon dioxide from fetus to mother

    • Endocrine function: secretes hormones such as estrogen, progesterone, and human chorionic gonadotropin; allows these hormones to diffuse from mother to fetus and fetus to mother

    • Immune function: provides passive immunity by transporting antibodies from the mother to the blood of the fetus

  14. The umbilical cord contains one vein and two arteries; the arteries transport oxygen-depleted blood from the fetus to the placenta. The vein transports oxygen-rich blood to the fetus. (Students can diagram the pathway to summarize the role).


Going Beyond

The development of the embryo is very complex and consists of much more detail than can be presented in this lesson. Remember that all parts of the fetus originally have developed from one fertilized cell. As cells divide and the organism grows, the cells differentiate and congregate to form all the specialized structures of the body. If you are interested in this, research how specific structures such as the eyes, ears, muscles, or bones are formed, to name a few.

Many other terms define specific processes that occur during fetal development. Research some of the following terms: organogenesis, neurogenesis, angiogenesis, chondrogenesis, myogenesis, osteogenesis, zygocity, and 'fourth germ layer'.

Another item that you may wish to research is the Carnegie Stages of development. You could learn how each stage is defined through the embryo's external and internal development. Include in your research how embryonic cells communicate with each other.