人体的发生过程为:精子和卵子受精,形成合子,合子卵裂成卵裂球,于受精后72 h形成桑椹胚,并于受精后5～7 d成为囊胚。囊胚由滋养层和内细胞群组成,滋养层将分化成胎盘及胎儿附属结构;内细胞群则再分化为内胚层、中胚层和外胚层。内胚层分化发育成胎儿的消化道、呼吸道上皮和腺体 The process of human body is: sperm and egg fertilization, the formation of zygotes, zygotes cleavage into blastomeres, morula formed after 72 h of fertilization, and 5-7 d after fertilization as a blastocyst. The blastocyst consists of a trophoblast and an inner cell mass. The trophoblast will differentiate into placenta and fetal accessory structures. The inner cell mass will then differentiate into endoderm, mesoderm, and ectoderm. Endodermal differentiation into the fetal digestive tract, respiratory epithelium and glands