apch47out animal development

apch47out animal development - Chapter 47 Animal...

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Chapter 47 Animal Development Lecture Outline Overview: A Body-Building Plan for Animals From egg to organism, an animal’s form develops gradually. The question of how a zygote becomes an animal has been asked for centuries. As recently as the 18th century, the prevailing idea was preformation, the notion that an egg or sperm contains an embryo that is a preformed miniature adult. The competing theory is epigenesis, proposed 2,000 years earlier by Aristotle. ° According to epigenesis, the form of an animal emerges from a relatively formless egg. As microscopy improved in the 19th century, biologists could see that embryos took shape in a series of progressive steps. ° Epigenesis displaced preformation as the favored explanation among embryologists. Both preformation and epigenesis have some legitimacy. ° Although the embryo’s form emerges gradually as it develops, aspects of the developmental plan are already in place in the eggs of many species. ° An organism’s development is primarily determined by the genome of the zygote and also by differences that arise between early embryonic cells. ° These differences set the stage for the expression of different genes in different cells. In some species, early embryonic cells become different because of the uneven distribution within the unfertilized egg of maternal substances called cytoplasmic determinants. ° These substances affect development of the cells that inherit them during the early mitotic divisions of the embryo. In other species, the differences between cells are due to their location in the developing embryo. Most species establish differences between early embryonic cells by a combination of these two mechanisms. As development continues, selective gene expression leads to cell differentiation, the specialization of cells in structure and function. Along with cell division and differentiation, development involves morphogenesis, the process by which an animal takes shape. Concept 47.1 After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis Fertilization activates the egg and brings together the nuclei of sperm and egg. The gametes (egg and sperm) are both highly specialized cell types. Fertilization combines haploid sets of chromosomes from two individuals into a single diploid cell, the zygote. Another key function of fertilization is activation of the egg. ° Contact of the sperm with the egg’s surface initiates metabolic reactions within the egg that trigger the onset of embryonic development. Sea urchin fertilization has been extensively studied. Sea urchin egg and sperm encounter each other after the animals release their gametes into seawater. ° The jelly coat of the egg attracts the sperm, which swims toward the egg.
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This note was uploaded on 10/18/2010 for the course CSM 123 taught by Professor Yey during the Spring '10 term at Mindanao State University.

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apch47out animal development - Chapter 47 Animal...

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