In this chapter you will: Describe the acrosomal reaction; describe the cortical reaction; distinguish among meroblastic cleavage and holoblastic cleavage; compare the formation of a blastula and gastrulation in a sea urchin, a frog, and a chick; list and explain the functions of the extraembryonic membranes;. | Chapter 47 Animal Development Overview: A Body-Building Plan for Animals It is difficult to imagine That each of us began life as a single cell, a zygote A human embryo at approximately 6–8 weeks after conception Shows the development of distinctive features Figure 1 mm The question of how a zygote becomes an animal Has been asked for centuries As recently as the 18th century The prevailing theory was a notion called preformation Preformation is the idea that the egg or sperm contains an embryo A preformed miniature infant, or “homunculus,” that simply becomes larger during development Figure An organism’s development Is determined by the genome of the zygote and by differences that arise between early embryonic cells Cell differentiation Is the specialization of cells in their structure and function Morphogenesis Is the process by which an animal takes shape Concept : After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis Important events regulating development Occur during fertilization and each of the three successive stages that build the animal’s body Fertilization The main function of fertilization Is to bring the haploid nuclei of sperm and egg together to form a diploid zygote Contact of the sperm with the egg’s surface Initiates metabolic reactions within the egg that trigger the onset of embryonic development The Acrosomal Reaction The acrosomal reaction Is triggered when the sperm meets the egg Releases hydrolytic enzymes that digest material surrounding the egg The acrosomal reaction Sperm nucleus Sperm plasma membrane Hydrolytic enzymes Cortical granule Cortical granule membrane EGG CYTOPLASM Basal body (centriole) Sperm head Acrosomal process Actin Acrosome Jelly coat Egg plasma membrane Vitelline layer Fused plasma membranes Perivitelline space Fertilization envelope Cortical reaction. Fusion of the gamete membranes triggers an increase of Ca2+ in the egg’s cytosol, causing cortical . | Chapter 47 Animal Development Overview: A Body-Building Plan for Animals It is difficult to imagine That each of us began life as a single cell, a zygote A human embryo at approximately 6–8 weeks after conception Shows the development of distinctive features Figure 1 mm The question of how a zygote becomes an animal Has been asked for centuries As recently as the 18th century The prevailing theory was a notion called preformation Preformation is the idea that the egg or sperm contains an embryo A preformed miniature infant, or “homunculus,” that simply becomes larger during development Figure An organism’s development Is determined by the genome of the zygote and by differences that arise between early embryonic cells Cell differentiation Is the specialization of cells in their structure and function Morphogenesis Is the process by which an animal takes shape Concept : After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis .
