Ch. 37 - Chapter 37 Animal Development What Is Animal...

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Chapter 37: Animal Development
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What Is Animal Development? The process by which an animal proceeds from fertilized egg through adulthood and eventual death Animals demonstrate two possible forms of development: Indirect development Direct development
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Indirect Development Where the juvenile has different morphology from adult Most invertebrates and some vertebrates (amphibians) Adults make large numbers of eggs, each with small yolk (food reserve) An immature larva emerges from egg Metamorphosis produces sexually mature adults
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larva adult adult larva
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Direct Development When the juvenile resembles the adult form Reptiles, birds, mammals, and some invertebrates Newborn is a sexually immature version of adult Adults produce fewer offspring, but are more developed
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Reptiles, Birds, and Mammals Produce Similar Extraembryonic Membranes Reptiles, birds, and mammals produce amnionic eggs Embryo encased in protective shell and liquid-filled space Acts as “private pond” Allows embryo to be less dependent on the external environment for development
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Reptiles, Birds, and Mammals Produce Similar Extraembryonic Membranes Amnionic eggs contain 4 extra-embryonic membranes: Chorion: lines shell, involved with O2 and CO2 exchange with environment Amnion: encloses embryo in watery environment Allantois: surrounds and isolates wastes Yolk sac: contains stored food
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Reptile Mammal shell yolk sac allantois embryo amnion chorion
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How Does Development Proceed? Multistage process including 1. Cleavage 2. Gastrulation 3. The formation of adult structures 4. Sexual maturation
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Cleavage Begins the Process Clevage: Zygote divides mitotically without an increase in size a solid round of ball of cells is formed ( morula) The morula develops an internal cavity ( blastocoel) becomes a blastula
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Gastrulation Forms Three Tissue Layers An indentation ( blastopore) forms and cells fold inward to form 3 layers: Endoderm: inner layer; digestive and respiratory tracts Mesoderm: middle layer; muscle and skeleton Ectoderm : outer layer; epidermis, circulatory, and nervous system A 3-layer embryo ( gastrula ) forms
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Adult Structures Develop Organs form (organogenesis) from each embryonic cell later: Organs “sculpted” by programmed death of excess cells (1) some cells die unless they receive a chemical “survival signal” (only motor neurons that connect with muscles live) (2) some cells live unless they receive a chemical “death signal” (webbing in human toes)
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Sexual Maturation Is Controlled by Genes and the Environment Age and environment can stimulate hormone release and sexual development Nutrition and early sexual contact may accelerate maturation
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How Is Development Controlled?
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