Chapter 60

Summary		Questions		Media Resources
60.1 Fertilization is the initial event in development.
• Fertilization is the union of an egg and a sperm to form a zygote. It is accomplished externally in most fish and amphibians, and internally in all other vertebrates. • The three stages of fertilization are (1) penetration, (2) activation, and (3) nuclei fusion.		1.  What happens during each stage of fertilization?		• Sperm and Egg Anatomy •Fertilization
60.2 Cell cleavage and the formation of a blastula set the stage for later development.
• Cleavage is the rapid division of the newly formed zygote into a mass of cells, without any increase in overall size. • The cleavages produce a hollow ball of cells, called the blastula.		2. What is the difference between holoblastic cleavage and meroblastic cleavage? What is responsible for an embryo undergoing one or the other type of cleavage?
60.3 Gastrulation forms the three germ layers of the embryo.
•During gastrulation, cells in the blastula change their relative positions, forming the three primary cell layers: ectoderm, mesoderm, and endoderm. •In eggs with moderate or large amounts of yolk, cells involute down and around the yolk, through a blastopore or primitive streak to form the three germ layers.		3. What is an archenteron, and during what developmental stage does it form? What is the future fate of this opening in vertebrates? 4. How is gastrulation in amphibians different from that in lancelets?		•Art Quiz: Gastrulation-Mammal
60.4 Body architecture is determined during the next stages of embryonic development.
• Neurulation involves the formation of the notochord and dorsal hollow nerve tube. • The formation of the neural crest is the first developmental event unique to vertebrates. Most of the distinctive structures associated with vertebrates are derived from the neural crest.		5. What structure unique to chordates forms during neurulation? 6. What are the functions of the amnion, chorion, and allantois in birds and mammals?		•Human Extraembryonic Membranes •Cell Differentiation •Early Development •Development •Frozen Embryos •Extra-embryonic Membranes (1) •Extra-embryonic Membranes (2)
60.5 Human development is divided into trimesters.
• Most of the critical events in human development occur in the first month of pregnancy. Cleavage occurs during the first week, gastrulation during the second week, neurulation during the third week, and organogenesis during the fourth week. • The second and third months of human development are devoted to morphogenesis and to the elaboration of the nervous system and sensory organs. • During the last six months before birth, the human fetus grows considerably, and the brain produces large numbers of neurons and establishes major nerve tracts.		7. How does the placenta prevent menstruation during the first two months of pregnancy? 8. At what time during human pregnancy does organogenesis occur? 9. Is neurological growth complete at birth? 10. What hormone stimulates lactation (milk production)? What hormone stimulates milk ejection from the breast?		•Human Development •Hormones and Pregnancy • Critically Ill Newborns •Allometric Growth

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