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妊娠
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胚胎与胎儿发育
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胚胎
术语胚胎
(embryo，自希腊语：在……内膨胀)定义为生命第2-18周期间的生命发育有机体。在这个时期，胚胎从微渺的细胞团发育成约2.54厘米（1英寸）长的小毛毛。随着发育的进展，胎盘这个特殊的物质交换器官在胚胎和子宫之间开始生长。通过脐带，胚胎连接到胎盘。（宝宝出生后不久，胎儿的脐带仍然与胎盘相连，然后，胎盘从子宫娩出。由此，胎盘也称为胞衣。）胎盘有过滤和屏障的作用，它允许胚胎（随后为胎儿）从母亲的血液吸收养分和氧气和从自己的血液排除二氧化碳和其他代谢废物到母体。可是，与此同时，胚胎与母体的血液循环系统仍就相互分隔。
人体发育的第1个月里，胚胎看起来与其他任何高等动物相似，诸如象猫、狗或猪的胚胎。随后在第2月里，它慢慢呈现出人的特征。这时，他/她开始发育出可辨认的面部，也发育出上肢、下肢、手指和足趾。此时尽管还没有出现性分化（也就是男女的性器官外观上仍然相同），但在其两腿之间，性器官的雏形依稀可见。当所有正在发育的器官最后变得清晰可辨时，他/她告别了胚胎期而进入了胎儿期。 |
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Pregnancy
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Development of Embryo and Fetus
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The Embryo
The word embryo (Greek: swelling within) refers to the growing organism from the second to the eighth week of its life. During this time, it develops from a tiny cell cluster into a little growth of about 1 inch in length. As this development proceeds, the placenta, a special organ of interchange, begins to grow between the embryo and the uterus. The embryo is connected to the placenta by the umbilical cord. (Soon after the birth of the baby, its umbilical cord is still connected to the placenta which is then expelled from the uterus. For this reason, the placenta is also called the afterbirth.) The placenta acts as a filter and as a barrier. It allows the embryo (and later the fetus) to absorb food and oxygen from the woman's blood and to eliminate carbon dioxide and other waste from its own blood in return. At the same time, however, the two blood systems remain completely separate.
During the first month of its life, the human embryo looks like that of any other higher animal, such as a cat, dog, or pig, for example. Then, during the second month, it slowly assumes human features. It starts to develop a recognizable face, as well as arms, legs, fingers, and toes. Between its legs, the primitive beginnings of sexual organs become discernible, although they are still undifferentiated at this point (i.e., they are the same for both male and female). When the entire growing organism finally becomes clearly identifiable as human, it leaves the stage of the embryo and enters that of the fetus. |
