师：同学们，大家好，欢迎来到初中生物学北师大版八年级上册第19章第一节《人的生殖和发育》第二课时。本节学习目标如下：一、描述人的生殖和发育过程；二、说出胚胎发育的过程和营养供应；三、感受母亲珍爱生命的伟大。通过上节课的学习，我们知道，每个人生命发育的起点是受精卵，它由男性产生的精子和女性卵巢产生的卵细胞，通过受精作用形成。那么，这个具体的过程是怎样的呢？受精卵又是如何继续发育为一个胎儿的呢？这就是本节课要学习的内容。下面，让我们通过一个视频来看一看受精作用的过程，观看时，同学们要认真记笔记。
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师：Fertilization is the epic story of a single sperm facing incredible odds to unite with an egg and form a new human life. It is the story of all of us. During ejaculation, about 300 million sperm enter the vagina. Soon afterward, millions of them leave or flow out of the vagina or die in its acidic environment. However, many survive because of the protective elements provided in the seminal fluid. Next, the sperm pass through the cervix and the opening into the uterus. Usually, it remains tightly closed, but during ovulation, the cervix is open for a few days. The sperm swim through the cervical mucus, which becomes more watery for easier passage. Once inside the cervix, the sperm continue swimming toward the uterus, though many would die trying to make it through the mucus. Some sperm remain behind in the folds of the cervix, but they may later continue the journey as backup to the first group. Inside the uterus, muscular contractions assist the sperm on their journey toward the egg. However, immune system cells mistake the sperm for foreign invaders, destroying thousands more.
Next, half the sperm head for the empty fallopian tube while the other half head toward the tube containing the unfertilized egg. Now, only a few thousand sperm remain inside the fallopian tube. Tiny cilia push the egg toward the uterus. To continue, the sperm must surge against this motion to reach the egg. Some sperm get trapped in the cilia and die.
During this part of the journey, chemicals in the reproductive tract cause the membranes covering the heads of the sperm to change. As a result, the sperm become hyperactive, swimming harder and faster toward their destination. By the time the sperm reach the egg, only a few dozen of the original 300 million remain.
The egg is covered with a layer of cells called the corona radiata. The sperm push through this layer to reach the outer layer of the egg, the zona pellucida. When the sperm reach the zona pellucida, they attach to specialized receptors on the surface, which triggers their acrosomes to release digestive enzymes, enabling the sperm to burrow into the layer. Inside the zona pellucida is a narrow, fluid - filled space just outside the egg cell membrane. The first sperm to make contact will fertilize the egg. After a perilous journey and against incredible odds, a single sperm attaches to the egg cell membrane. Within a few minutes, their membranes break, fuse, and the egg pulls the sperm inside. This effect causes changes in the egg membrane that prevent other sperm from attaching to it. Next, the egg releases chemicals that push other sperm away from the egg and create an impenetrable fertilization membrane. As the reaction spreads outward, the zona pellucida hardens, trapping any sperm unlucky enough to be caught inside. Outside the egg, sperm are no longer able to attach to the zona pellucida.
Meanwhile, inside the egg, the tightly packed male genetic material spreads out. A new membrane forms around the genetic material, creating the male pronucleus. Inside, the genetic material reforms into 23 chromosomes. The female genetic material, awakened by the fusion of the sperm with the egg, finishes dividing, resulting in the female pronucleus, which also contains 23 chromosomes. As the male and female pronuclei form, spindle - like threads of microtubules crawl toward each other. The two sets of chromosomes join together, completing the process of fertilization. At this moment, a unique genetic code arises, instantly determining gender, hair color, and hundreds of other characteristics. This new single - celled zygote is the beginning of a new human being. And now, the cilia in the fallopian tube gently sweep the zygote toward the uterus, where it will implant in the rich uterine lining, growing and maturing for the next nine months until ready for birth.