Chapter Summary

• The sperm and the egg contribute differentially to the zygote. The sperm contributes a haploid nucleus and, in most species, a centriole. The egg contributes a haploid nucleus, nutrients, ribosomes, mitochondria, mRNAs, and proteins.

• In amphibians, the cytoplasmic contents of the unfertilized egg are not distributed homogeneously, and they are rearranged after fertilization to set up the major axes of the future embryo. The nutrient molecules are generally found in the vegetal hemisphere, whereas the nucleus is found in the animal hemisphere. Review Figures 43.1, 43.2

• Cleavage is a period of rapid cell division. Except in mammals, little if any gene expression occurs during cleavage. Cleavage can be complete or incomplete, and the pattern of cell divisions depends on the orientation of the mitotic spindles. The result of cleavage is a ball or mass of cells called a blastula. Review Figure 43.3

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• Early cell divisions in mammals are unique in being slow and allowing for gene expression early in the process. These cell divisions produce a blastocyst composed of an inner cell mass that becomes the embryo and an outer cell mass that develops as the trophoblast. At the time of implantation, the trophoblast secretes molecules that help the blastocyst implant in the uterine wall. Review Figures 43.4, 43.5

• A fate map can be created by labeling specific blastomeres and observing what tissues and organs are formed by their progeny later in development. Review Figure 43.6

• Some species undergo mosaic development, in which the fate of each cell is determined during early divisions. Other species, including vertebrates, undergo regulative development, in which remaining cells can compensate for cells lost in early cleavages.

• Gastrulation involves massive cell movements that produce three germ layers and place cells from various regions of the blastula into new associations with one another. Review Figure 43.7, Animation 43.1

• The initial step of sea urchin and amphibian gastrulation is inward movement of certain blastomeres. The site of inward movement becomes the blastopore. Cells that move into the blastula become the endoderm and mesoderm; cells remaining on the outside become the ectoderm. Cytoplasmic factors in the vegetal pole cells are essential to initiate development. Review Figure 43.7, Focus: Key Figure 43.8

• The dorsal lip of the amphibian blastopore is a critical site for cell determination. It has been called the primary embryonic organizer, or organizer, because it induces determination in cells that pass over it during gastrulation. Review Figures 43.8–43.10, Animation 43.2

• The protein β-catenin activates a signaling cascade that induces the primary embryonic organizer and sets up the anterior–posterior body axis. Review Figures 43.2

• Gastrulation in reptiles, including birds, differs from that in sea urchins and frogs because the large amount of yolk causes the blastula to form a flattened disc of cells. Review Figure 43.11

• Although their eggs have no yolk, placental mammals have a pattern of gastrulation similar to that of reptiles and birds.

• The direction of extracellular fluid flow over the ventral surface of the node breaks bilateral symmetry in the mammalian gastrula. Review Investigating Life: Does the Direction of Nodal Flow Influence the Development of Left–Right Asymmetry in the Mouse Embryo?

• Gastrulation is followed by organogenesis, the process where tissues interact to form organs and organ systems.

• In the formation of the vertebrate nervous system, one group of cells that migrates over the blastopore lip is determined to become the notochord. The notochord organizes the overlying ectoderm to thicken, form parallel ridges, and fold in on itself to form a neural tube below the epidermal ectoderm. The nervous system develops from this neural tube. Review Figure 43.12

• The notochord and neural crest cells participate in the segmental organization of mesoderm into structures called somites along the body axis. Rudimentary organs and organ systems form during these stages. Review Figure 43.13

• In vertebrates, Hox genes determine the pattern of anterior–posterior differentiation along the body axis. Other genes, such as sonic hedgehog, contribute to dorsal–ventral differentiation. Review Figure 43.14

• The embryos of reptiles and mammals are protected and nurtured by four extraembryonic membranes. In reptiles the yolk sac surrounds the yolk and provides nutrients to the embryo, the chorion lines the eggshell and participates in gas exchange, the amnion surrounds the embryo and encloses it in an aqueous environment, and the allantois stores metabolic wastes. Review Figure 43.15, Activity 43.1

• In mammals the chorion and the trophoblast cells interact with the maternal uterus to form a placenta, which provides the embryo with nutrients and gas exchange. The amnion encloses the embryo in an aqueous environment. Review Figure 43.16

• Human pregnancy, or gestation, lasts 9 months. The embryo forms in the first trimester; during this time, it is most vulnerable to environmental factors that can lead to birth defects. During the second and third trimesters the fetus grows, external features form, and the organ systems mature. Review Figure 43.17