Chapter 3
Basic Embryology
I. The Basic Body Plan (p. 51, Figs. 3.1–3.2)
A. Embryology is the prenatal study of the origin and development of an individual.
1. The two stages of prenatal development are the embryonic period and the fetal period.
B. The tube-within-a-tube basic plan is evident by month 2.
1. The basic structures present are precursors for the skin, trunk muscles, vertebral column, spinal cord and
brain, digestive and respiratory tubes, serous cavities, heart, kidneys, gonads, and limbs.
II. The Embryonic Period (p. 51, Figs. 3.3–3.11)
A. Week 1 is characterized by fertilization, a 4-celled stage, the morula, and implantation of the blastocyst.
B. In week 2, the two-layered embryo completes implantation and forms the bilaminar embryonic; the amnion and
yolk sac also form.
C. During week 3, the embryo becomes a trilaminar disc.
1. The endoderm, ectoderm, and mesoderm are established
2. The primitive streak and notochord form
3. Neurulation begins
4. The mesoderm begins to differentiate
D. The flat embryo folds into a tadpole shape in week 4 achieving the basic plan of a tube-in-a-tube; limb buds
appear.
E. During weeks 5–8 the major adult structures are derived from ectoderm, endoderm, and mesoderm. (Fig. 3.9)
III. The Fetal Period (pp. 61–65, Fig. 3.12, Table 3.1)
A. Weeks 9 through 38 are a time of maturation and rapid growth.
SUPPLEMENTAL STUDENT MATERIALS to Human Anatomy,
Fifth Edition
Chapter 3: Basic Embryology
To the Student
Knowledge of basic embryological concepts will enable you to understand the origin of normal adult anatomy and to
understand the origin of birth defects. The central challenge of this chapter is to understand how the adult-like body
components of a fetus are assembled from a single fertilized ovum.
Step 1: Distinguish between an embryo and a fetus.
- Define embryo.
- Define fetus.
Step 2: Outline early stages of development from zygote to blastocyst.
- Describe the process of fertilization.
- Define zygote.
- Relate the anatomy of the uterine tube to the movement of an embryo within it.
- Describe the process of implantation. Be able to discuss the events depicted in Figure 3.4a–e and name the
embryonic tissue structures that form following implantation.
Step 3: Summarize week 2 through week 8 of embryonic development.
- Describe how the embryo becomes a two-layered disc during week 2. (Figure 3.6)
- Describe the formation of three germ layers during week 3. (Figure 3.7)
- Explain the formation of the neural tube, notochord, and neural crest. (Figure 3.8)
- Explain how the embryo changes shape during week 4. (Figure 3.9)
- List examples of the main derivatives of the embryonic germ layers.
- Describe the main events of the second month of development.
Step 4: List the major events of fetal development.
- Summarize monthly changes in the fetus until birth. (Table 3.1)