Section outline

  • Depiction of an embryo showing developing features like eyes and limb buds.

    At one time, did we really all look alike?
    ::有一次,我们都长得真像吗?

    We all start as a single and soon grow into an embryo . Notice the remarkable details beginning to form. The , backbone , and limb buds are obvious. Think about the amazing complexity that must be going on inside the embryo, and the tremendous amount of growth and still to come. So, yes, at one time we all looked similar.
    ::我们都从一个单一的胚胎开始,并很快成长为胚胎。注意到这些惊人的细节开始形成。骨干和肢体芽是显而易见的。想想胚胎中必须经历的惊人的复杂性,以及巨大的成长和未来的发展。所以,是的,我们一度看起来都相似。

    Growth and Development of the Embryo
    ::安布里约的增长和发展

    After implantation occurs, the blastocyst is called an embryo . The embryonic stage lasts through the eighth week following . During this time, the embryo grows in size and becomes more complex. It develops specialized cells and tissues and starts to form most organs . 
    ::植入后,甲草胺被称为胚胎,胚胎阶段持续到随后的第8周。在这一时期,胚胎的体积会变小,变得更复杂,形成专门的细胞和组织,并开始形成大多数器官。

    Formation of Cell Layers
    ::单元格层形成

    During the second week after fertilization, cells in the embryo migrate to form three distinct cell layers, called the ectoderm , mesoderm , and endoderm . Each layer will soon develop into different types of cells and tissues, as shown in Figure .
    ::在受孕后第二周,胚胎中的细胞会迁移到三个不同的细胞层,称为乳头细胞、中位细胞和内代细胞。 每个细胞层很快会发展成不同种类的细胞和组织,如图6所示。

    Cell layers in a 2-week-old embryo showing ectoderm, mesoderm, and endoderm development.

    Cell Layers of the Embryo. The migration of cells into three layers occurs in the 2-week-old embryo. What organs eventually develop from the ectoderm cell layer? Which cell layer develops into muscle tissues?
    ::Emblyo 细胞层。 细胞向三层的迁移发生在两周大的胚胎中。 哪些器官最终会从乳腺细胞层中产生? 哪一个细胞层会发展成肌肉组织?

    Differentiation of Cells
    ::单元格的差别

    A zygote is a single cell. How does a single cell develop into many different types of cells? During the third week after fertilization, the embryo begins to undergo cellular differentiation . Differentiation is the process by which unspecialized cells become specialized. As illustrated in Figure , differentiation occurs as certain genes are expressed ("switched on") while other genes are switched off. Because of this process, cells develop unique structures and abilities that suit them for their specialized functions. 
    ::zygote 是一个单细胞。 单细胞是如何发展成多种不同类型的细胞的? 在受孕后的第三周,胚胎开始接受细胞差异化。 差异化是非专门化细胞成为专业化细胞的过程。 如图所示, 差异化会随着某些基因的表达( “ 开关 ” ) 而产生, 而其他基因则被关闭。 由于这一过程, 细胞发展出适合它们特殊功能的独特结构和能力。

    Cells with identical DNA differentiate into blood, nerve, and muscle cells through gene expression.

    Cellular differentiation occurs in the 3-week-old embryo.
    ::3周大的胚胎存在细胞差异。

    Organ Formation
    ::组织形成

    After cells differentiate, all the major organs begin to form during the remaining weeks of embryonic development. A few of the developments that occur in the embryo during weeks 4 through 8 are listed in Figure . As the embryo develops, it also grows in size. By the eighth week of development, the embryo is about 30 millimeters (just over 1 inch) in length. It may also have begun to move.
    ::在细胞区分之后,所有主要器官在胚胎发育剩余的几周内开始形成,图中列出了胚胎4至8周中出现的一些发育情况。随着胚胎的发育,胚胎的体积也在扩大,到发育的第八周,胚胎长度约为30毫米(略高于1英寸),也可能已经开始移动。

    Embryonic development stages from 4 to 8 weeks, showcasing organ formation and growth.

    Embryonic Development (Weeks 4–8). Most organs develop in the embryo during weeks 4 through 8. If the embryo is exposed to toxins during this period, the effects are likely to be very damaging. Can you explain why? (Note: the drawings of the embryos are not to scale.)
    ::细胞发育(Weeks 4-8) 。 大部分器官在4周到8周期间在胚胎中发育。 如果胚胎在此期间暴露在毒素之下,那么其影响很可能非常有害。 你能解释原因吗? (注:胚胎的图画不会缩放。 )

       

     

    Summary
    ::摘要

    • The embryonic stage begins with implantation.
      ::胚胎阶段从植入开始
    • An embryo forms three distinct cell layers, and each layer develops into different types of cells and organs.
      ::胚胎形成三个不同的细胞层,每个细胞层发展成不同类型的细胞和器官。

    Review
    ::回顾

    1. Explain how the embryo forms specialized cells.
      ::解释胚胎是如何形成特殊细胞的
    2. What organs eventually develop from the ectoderm cell layer?
      ::哪些器官最终会从乳腺细胞层中产生?
    3. Which cell layer develops into muscle tissues?
      ::哪个细胞层发展成肌肉组织?
    4. If the embryo is exposed to toxins during weeks 4 through 8, the effects are likely to be very damaging. Can you explain why?
      ::如果胚胎在第4周至第8周接触毒素,其影响很可能非常有害。你能解释为什么吗?