n the upper third of the oviduct. Thirty minutes after ejaculation, sperm are present in the oviduct, having traveled from the vagina through the uterus and into the oviduct. Sperm traverse this distance by the beating of their flagellum. Of the several hundred million sperm released in the ejaculation, only a few thousand reach the egg. One reason is that the fluids secreted by the vagina are acidic and destroy most of the delicate sperm. Some sperm survive because of the buffering effect of semen. The surviving sperm swims up the vagina and into the uterus. Only a few hundred sperm pass into the two the fallopian tubes. The egg is in one of the oviducts.The head of the sperm contains enzymes that help it penetrate the egg. The sperm will lose its midpart and its tail as it penetrates the egg. After one sperm penetrates the egg, the eggs membrane changes it electrical charge. That prevents the other sperm from getting in the egg. Also its nucleus combines with the eggs nucleus to form a zygote. The fertilized egg moves to the uterus. As the zygote passes down the oviduct, it begins to divide by mitosis. They zygote obtains nutrients from fluids secreted by the mother during its journey. The zygote passes into the uterus by the sixth day. Continuous cell divisions cause a hollow ball of cells called a blastocyst to form. After seven to eight days after fertilization the blastocyst attaches to the uterine lining. Implantation is the attachment of the blastocyst to the lining of the uterus. A small inner mass of cells within the blastocyst will soon become a human embryo. The uterine lining becomes enlarged and prepared for implantation of the embryo in the trophoblast layer. Twelve days after fertilization, the trophoblast has formed a two-layered chorion. Human chrionic gonadotropin is secreted by the chorion, and prolongs the life of the corpus luteum until the placenta begins to secrete estrogen and progesterone. ...
