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After studying this chapter, you should be able to:

  • Describe the major events of the first week of development, including fertilization, blastocyst formation, and implantation.

  • Trace the development of the trophoblast, noting its roles in implantation and establishing uteroplacental circulation.

  • Trace the formation and eventual fates of developmental cavities, including the amniotic cavity, chorionic cavity, and yolk sac.

  • Trace the development and folding of the embryonic disc, noting the formation of significant structures such as the notochord, neural tube, and somites.

  • Relate the spatial arrangement of the developmental structures that are present at the conclusion of embryonic folding to their mature derivatives and the overall body organization.



  • Fertilization occurs shortly after ovulation, when haploid gamete cells (i.e., the oocyte and sperm) combine to form a diploid zygote (Figure 1-1, Clinical Box 1-1).

    • This is the beginning of the first week of development according to fertilization age, which counts 38 weeks (266 days) from fertilization until birth. This textbook uses fertilization age to track developmental progress.

    • This is the beginning of the third week according to gestational age, which counts 40 weeks (280 days) from the first day of the last normal menstrual period until birth. Gestational age relies on menstruation as a useful time point because the exact date of fertilization generally is not known.

  • Fertilization typically takes place in the ampulla of the oviduct (uterine tube).

Figure 1-1.

Week 1: Fertilization through implantation. Fertilization typically occurs in the ampulla of the uterine tube, creating a zygote. The zygote then undergoes repeated cleavage divisions while being propelled toward the uterine cavity. When the conceptus has 16–32 cells, it is called a morula. The morula undergoes cavitation to become a blastocyst—with a blastocyst cavity, trophoblast, and embryoblast—which will implant in the endometrium.

Source: Reproduced, with permission, from Michael P. McKinley, Valerie Dean O’Loughlin, Elizabeth E. Pennefather-O’Brien. Chapter 3, Embryology. Human Anatomy, 6e. McGraw Hill. 2021.

CLINICAL BOX 1-1. Abnormal Zygotes

  • An estimated 50 percent of conceptions end in spontaneous abortion due to abnormalities in the zygote.

    • In about half of these cases, chromosomal abnormalities are the cause of termination.

  • These pregnancies are often lost within two or three weeks of fertilization, before pregnancy is detected. As a result, their prevalence is difficult to measure accurately.

Division, Transport, and Cavitation

  • The zygote undergoes a series of cleavage divisions (mitotic cell divisions without growth in size), producing cells called blastomeres, all still contained within the zona pellucida (the glycoprotein shell that surrounded the oocyte) (Figure 1-1). When the cluster of cells contains 16–32 cells, it is known as a morula.

  • During these divisions, ...

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