Germdiscformanddifferentiate胚层的形成和分化1、Developmentofthebilaminargermdisc（二胚层的形成,2w）：(1)二胚层胚盘（bilaminargermdisc)。上胚层（epiblast）下胚层（hypoblast）羊膜腔（amnioticcavity)，卵黄囊(yolksac)。体蒂(3)Theformationoftheyolksac（卵黄囊形成）下胚层形成次级卵黄囊(4)Thehypoblastgivesrisetotheextraembryonicendodermthatlinestheprimaryyolksac(5)Thechorioniccavityisproducedinconjunctionwiththedevelopmentoftheextraembryonicmesoderm(6)Thedefinitiveyolksacisformedbyanewwaveofcellsthatmigratefromthehypoblastanddisplaceheuser’smembrane2、三胚层的形成(第3周）(1)Theprimitivestreakappearsatthebeginningofthethirdweek(原条的出现,3w)原条primitivestreak原结primitivenode原凹primitivepit原沟primitivegroove口咽膜(2)Thedefinitiveendoderm（内胚层）andintraembryonicmesoderm（胚内中胚层）formthroughtheprimitivestreakTheprimitivestreakretreatsanddisappears,andthecaudaleminencegivesrisetocaudalstructuresofthebody.（15-26day）ThemechanicsofgastrulationarenotwellunderstoodThefateofgastrulationepiblastcellscanbepredictedfromtheirsiteoforigin.Studymethod:Celltracingandcelllineage（细胞系）studies(3)Thenotochord(脊索)isproducedbycellsthatingressthroughtheprimitivenodePrimitivepot(原结）TheNotochordalprocess（脊索突）thenotochordalplate（脊索板）thenotochord（脊索)意义：决定胚体中轴诱导外胚层形成神经板椎间盘中髓核。Theultimatefateofthenotochordisdebated.Mesodermisexcludedfromthebuccopharyngealandcloacalmembrances.小结：胚胎发育：第一周：植入第二周：二胚层第三周：三胚层Meningohydroencephalocele(脑积水脑膜脑突出）Thisanomalymayformthroughdefectiveossificationoftheoccipitalboneandprobablyisnotaconsequenceofdefectiveclosureoftheneuraltube.Headlessmice.Thedevelopmentandmaintenanceofthe“headorganizingcenter”requirestheexpressionofthehomeoboxgeneLim-1.KnockoutofLim-1resultsinheadlessmice.小结：神经系统、脑垂体、松果体、皮肤：表皮、附属结构凹陷上皮：口、鼻、肛门、外耳道、膜迷路、角膜上皮、视网膜、晶状体肾上腺髓质、腮腺上皮2、mesodermdifferentiate(中胚层的分化)：（1）paraxialmesoderm(轴旁中胚层)——(somites)体节，第5周全部形成，共42-44对，分化为真皮，大部分中轴骨和骨骼肌。（2）intermediatemesoderm(间介中胚层)——位于体节外侧，泌尿生殖系统发生的原基。3、Endodermdifferentiate(内胚层的分化)：随胚体的卷曲，内胚层形成原始消化管，内胚层：是消化系统、呼吸系统和中耳、甲状腺、甲状旁腺、扁桃体、胸腺，膀胱和尿道等上皮的原基。五、胚胎外形的建立及演变1、胚体的形成五周鳃弓