2016_【高级精品】METS3考试须知.docVIP

医护英语水平考试（三级）辅导 METS LEVEL III UNIT ONE TEXT Prenatal Development Normally, it takes about 40 weeks for a single cell to grow up into a fully formed newborn baby. The long period takes on dramatic changes and is typically divided into three phases: the zygote phase, the embryo phase and the fetus phase. The zygote refers to the single-celled form of the new baby produced by fertilization. It travels along the mother’s fallopian tube and finally embeds itself into the uterus. At the same time, the zygote divides rapidly and turns into the multi-celled form, called blastula. After the first three weeks of zygote phase follows the embryo phase. Dramatic changes and organ and system developments take place during this phase. The placenta comes into being and establishes the life-support system between the embryo and the mother. It is responsible for gas and blood supply and waste excretion of the new baby. Meanwhile, part of the blastula differentiates into the amniotic sac, which contains amniotic fluid and the baby. The main part of embryo differentiates into three layers. The outside layer, called ectoderm, eventually becomes the spinal column, the nervous system and the skin. The middle layer, named mesoderm, later develops into muscle and bone. The inner layer, named endoderm, differentiates into the internal organs. During the fourth and fifth weeks after conception, heartbeat can be detected and limbs, hands or even fingers apparent. Vital organs, including brain, lungs, stomach and kidneys are formed and start functioning in the following weeks. By the end of the ninth week, testes or ovaries are visible to tell the sex of the baby. The last phase of fetus begins from the 12th week after conception. The baby with fully developed organs and systems comes into the rapid growth period. Various kinds of movements are detected by the 14th week, including swallow, digestion, urination and limb movements. In the later phase of fetus development, body size and weight