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妊娠 |
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胚胎與胎兒發育
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胚胎
術語胚胎
(embryo，自希臘語：在……內膨脹)定義為生命第2-18周期間的生命發育有機體。在這個時期，胚胎從微渺的細胞團發育成約2.54釐米（1英寸）長的小毛毛。隨著發育的進展，胎盤這個特殊的物質交換器官在胚胎和子宮之間開始生長。通過臍帶，胚胎連接到胎盤。（寶寶出生後不久，胎兒的臍帶仍然與胎盤相連，然後，胎盤從子宮娩出。由此，胎盤也稱為胞衣。）胎盤有過濾和屏障的作用，它允許胚胎（隨後為胎兒）從母親的血液吸收養分和氧氣和從自己的血液排除二氧化碳和其他代謝廢物到母體。可是，與此同時，胚胎與母體的血液循環系統仍就相互分隔。
人體發育的第1個月裏，胚胎看起來與其他任何高等動物相似，諸如象貓、狗或豬的胚胎。隨後在第2月裏，它慢慢呈現出人的特徵。這時，他/她開始發育出可辨認的面部，也發育出上肢、下肢、手指和足趾。此時儘管還沒有出現性分化（也就是男女的性器官外觀上仍然相同），但在其兩腿之間，性器官的雛形依稀可見。當所有正在發育的器官最後變得清晰可辨時，他/她告別了胚胎期而進入了胎兒期。 |
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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. |
