胚胎

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胚胎是专指有性生殖而言，是指雄性生殖细胞和雌性生殖细胞结合成为合子之后，经过多次细胞分裂和细胞分化后形成的有发育成生物成体的能力的雏体。它指的是发育生物学最早的阶段。

[编辑] 简述

有性繁殖的生物体里，一旦精子使卵子受孕，卵子就变成受精卵，并同时拥有精子和卵子的DNA。植物、动物、部分原生物中，受精卵会自发细胞分裂，并形成一个多细胞的生物体。胚胎指的就是这个发展形成过程的最初阶段，从受精卵开始第一次分裂，到下一阶段发展开始前。

[编辑] 资料

植物: In 植物学, a 种子植物 embryo is part of a 种子, consisting of precursor tissues for the leaves, stem (see hypocotyl), and root (see radicle), as well as one or more cotyledons. Once the embryo begins to germinate — grow out from the seed — it is called a seedling. Plants that do not produce seeds, but do produce an embryo include the 苔藓类植物和蕨类植物. In these plants, the embryo is a young plant that grows attached to a parental gametophyte.
动物: The embryo of a placental mammal is defined as the organism between the first division of the zygote (a fertilized ovum) until it becomes a 胎儿. In humans, the embryo is defined as the product of conception from implantation in the uterus through the eighth week of development. An embryo is called a fetus at a more advanced stage of development and up until birth or hatching. In humans, this is from the eighth week of gestation. In other vertebrate animals, the embryo develops inside an egg. Invertebrate animals also develop from an embryo.

[编辑] 人类胚胎的发育

1-4星期 After conception, the embryo will begin to "search" for a place to attach to the wall of the uterus (endometrium). When it comes into contact with the endometrium and implants itself there, the connections between the mother and the embryo will begin to form, including the umbilical cord. The embryo's growth centers around an axis, which will become the spine and spinal cord.

5-6星期 Chemicals produced by the embryo stop the woman's menstrual cycle. The brain begins to develop, showing brain wave activity at about the 6th week. The heart will begin to beat around the same time. Stubs begin to be visible where arms and legs will grow later. All of the main organs begin to grow. The head represents about one half of the embryo's axial length, and more than half of the embryo's mass.

7-8星期 胚胎的血型基本定型。 Embryo is capable of motion, and the eyes begin to form. Most organs have developed or have begun developing. At the end of the 8th week, the embryonic stage is over, and the fetal stage begins.