A renowned developmental biologist, Lewis Wolpert, once observed that birth, death, and marriage are not the most important events in human life—rather, Wolpert said, the most important moment in life is gastrulation. Given the discussion in Section 17.1, what do you think he meant?

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Section 17.1 The fertilization of an oocyte by a sperm launches several key stages in early development. During cleavage, the fertilized egg undergoes cell divisions that form the early multicellular embryo. The destiny of various cell lines is established in part by the portion of cytoplasm inherited at this time. Gastrulation lays out the organizational framework of the whole body. Endoderm, ectoderm, and mesoderm form; all the tissues of the adult body will develop from these three germ layers. Differentiation is the process during which cells come to have specific structures and functions. In morphogenesis, tissues form and become organized into organs. Tissues and organs continue to mature as the fetus develops and even after birth.