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1、Chapter3PrenatalDevelopmentStagesGerminalEmbryonicFetalGerminalStagePeriodfromconceptiontoimplantation(approximately2weeks)ZygoterepeatedlydividesbutdoesnotgaininmassBeginsabout36hoursafteritisformedBlastocyst–CelldifferentiationInnerpartofblastocyst–embryonicdiskOut
2、erpartoftheblastocyst-trophoblastFigure3.1EarlyDaysoftheGerminalStageNourishmentDuringtheGerminalStagePriortoimplantation,thedividingcellsarenourishedbytheyolkoftheoriginaleggOnceimplanted,nourishmentisobtainedfromthemotherFailedPregnancyMiscarriage(spontaneousaborti
3、on)UsuallystemsfromabnormalitiesindevelopmentOne-thirdofallpregnanciesresultinmiscarriageFertilization/PrenatalPeriod152814EmbryonicStageBeginswithimplantation(3rdweek)through8thweekOrganogenesisMajororgansystemsdifferentiateDevelopmentaltrendsCephalocaudal–headtakes
4、precedenceoverlowerpartsofthebodyProximodistal–centralaxisofbodyoutwardFigure3.2HumanEmbryosandFetusesEmbryonicStageEmbryonicdiskEctoderm(outerlayer)developsintonervoussystem,sensoryorgans,nails,hair,teeth,andskinEndoderm(innerlayer)formsthedigestiveandrespiratorysys
5、tems,liver,andpancreasMesoderm(middlelayer)developsintotheexcretory,reproductiveandcirculatorysystem,muscles,skeleton,andinnerlayerofskinNeuraltube–developsintobrainandspinalcordImportantDevelopmentsinEmbryonicStageHeartbeginstobeat(3rdweek)Armbudsandlegbudsappear(4t
6、hweek)Eyes,ears,noseandmouthbegintotakeshapeLimbsareelongating,facialfeaturesbecomedistinct(8thweek)Teethbuds,workingkidneysandliver(8thweek)DevelopmentoftheNervousSystemByendoffirstmonth,neuraltubeisproducing400millionneuronsperdayNeuronsformcerebralhemispheres(5thw
7、eek)Cellsinthenervoussystembegintoreleaseneurotransmitters(2ndmonth)SexualDifferentiationNondescriptsexorgansformed(5to6weeks)InternalandexternalgenitalsatthisstageresemblefemalestructuresMüllerian(female)ductsandWolffian(male)ductsSexorgansbegintodifferentiate,based
8、ongeneticcode(7thweek)YchromosomecausestestestodifferentiateNoYchromosomecausesovariestodifferentiateDistinctexternalgenitalstructu
