Читать книгу Child Development From Infancy to Adolescence - Laura E. Levine - Страница 257

During ovulation, a woman’s ovary releases an egg or ovum. If the ovum is fertilized by a sperm, the resulting zygote multiplies until it becomes a blastocyst, which implants in the lining of the uterus. This is the germinal stage, from conception to 2 weeks postconception. In the embryonic stage, from 2 weeks to 2 months postconception, the cells increase to form an inner cell mass that becomes the embryo, and a ring of cells that becomes the support system, including the placenta, the amnion, and the chorion. During organogenesis, all the major organ systems are laid down. This is a critical period in development, and the developing embryo is vulnerable to teratogens, which can disrupt the developmental process. Development proceeds in a cephalocaudal direction. In the third stage, the fetal stage, from 2 months to birth, the fetus grows in size and weight, and all the organ systems become functional prior to birth.