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1、医护英语水平考试（三级）辅导METSLEVELIIIUNITONETEXTPrenatalDevelopmentNormally,ittakesabout40weeksforasinglecelltogrowupintoafullyformednewbornbaby.Thelongperiodtakesondramaticchangesandistypicallydividedintothreephases:thezygotephase,theembryophaseandthefetusphase.Thezygotereferstothesingle-celledformofthene
2、wbabyproducedbyfertilization.Ittravelsalongthemother’sfallopiantubeandfinallyembedsitselfintotheuterus.Atthesametime,thezygotedividesrapidlyandturnsintothemulti-celledform,calledblastula.Afterthefirstthreeweeksofzygotephasefollowstheembryophase.Dramaticchangesandorganandsystemdevelopmentstakepla
3、ceduringthisphase.Theplacentacomesintobeingandestablishesthelife-supportsystembetweentheembryoandthemother.Itisresponsibleforgasandbloodsupplyandwasteexcretionofthenewbaby.Meanwhile,partoftheblastuladifferentiatesintotheamnioticsac,whichcontainsamnioticfluidandthebaby.Themainpartofembryodifferen
4、tiatesintothreelayers.Theoutsidelayer,calledectoderm,eventuallybecomesthespinalcolumn,thenervoussystemandtheskin.Themiddlelayer,namedmesoderm,laterdevelopsintomuscleandbone.Theinnerlayer,namedendoderm,differentiatesintotheinternalorgans.Duringthefourthandfifthweeksafterconception,heartbeatcanbed
5、etectedandlimbs,handsorevenfingersapparent.Vitalorgans,includingbrain,lungs,stomachandkidneysareformedandstartfunctioninginthefollowingweeks.Bytheendoftheninthweek,testesorovariesarevisibletotellthesexofthebaby.Thelastphaseoffetusbeginsfromthe12thweekafterconception.Thebabywithfullydevelopedorga
6、nsandsystemscomesintotherapidgrowthperiod.Variouskindsofmovementsaredetectedbythe14thweek,includingswallow,digestion,urinationandlimbmovements.Inthelaterphaseoffetusdevelopment,bodysizeandweightaredramaticallyincreasedtillbirth.Sensoryperceptionsaredevelopedbetweenthe13thand29thweeks.Thefetusisa
7、lerttotheoutsidenoise,sensitivetodifferenttastesandabletoinitiateneurologicalreactionstostresshormones.Therefore,pregnantwomenoftencomplainofthevigorouskickingbythefetusespeciallywhenthemothersthemselvesfallintodistress.NewW
