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Chapter 8

Life on Earth Animals Chapter 8 Notes.docx

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Wilfrid Laurier University
Cristina Gheorghiu

01162013The primary organizerAlso known as the Spemann organizerDiscovered by Spemann when he used tissue from salamanders to induce development in another egg the dorsal lip tissue specificallyModern Discovery This occurs by secretion of certain molecules that trigger of repress the activity of combinations of genes in near by cellsEarly Concepts Preformation Versus EpigensisPreformation Belief that young animals were preformed in eggs and that development was simply a matter of unfolding what was already thereththConcept strongly supported by most 17 and 18 century naturalist philosophers Eginesis1759 Kasper Fredrich WolffShowed in chickens no preformation but granulated undifferentiated material that became arranged in layersTermed Epigensis meaning an idea that a fertilized egg contains building material only somehow assembled by an unknown directing forcePresently know far more about what directs growth and differentiation Hierarchy of Developmental decisions the sequential emergence of cellular diversityThe decision made at each stage in the hierarchy further limits developmental fateOnce cells commit to differentiation it is said to be determined and usually irreversible2 basic processes responsible for progressive subdivision are cytoplasmic specification and inductionFertilization The union of male and female gametes to form a zygoteAccomplishes 2 thingsiProvides recombination of parental genesiiRestores original diploidof chromosomes characteristic of a species and activates the egg to begin developmentPolar bodiescells containing a haploid nucleus and very little cytoplasm they eventually degenerateArtificial Parthenogenesis eggs of some species can be artificially induced to initiate development without spermIn most species embryo cannot progress far in development without lethal developmental abnormalities arising
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