For fertilization to occur, sperm cells must be released in the vagina during the period that the egg cell is alive. The sperm cells move through the uterus into the Fallopian tube, where one sperm cell may fertilize the egg cell. The fertilization brings together 23 chromosomes from the male and 23 chromosomes from the female, resulting in the formation of a fertilized egg cell with 46 chromosomes. The fertilized cell is a zygote.
The zygote undergoes mitosis to form two identical cells that remain attached. This takes place about 36 hours after fertilization. Mitosis then occurs more frequently. Soon a solid ball of cells, a morula, results. Morula formation occurs about six days after fertilization. During that time the cells are moving through the Fallopian tube. Within the next two days, a hollow ball of cells called a blastocyst forms. The blastocyst enters the uterus. At one end of the blastocyst, a group of cells called theinner cell mass continues to develop.
About eight days after fertilization, the blastocyst implants itself in the endometrium of the uterus. During implantation, the outer cells take root in the endometrium. This outer layer of cells, called the trophoblast, gives rise to projections that form vessels. These vessels merge with the maternal blood vessels to form the placenta. The trophoblast also develops into three membranes: the amnion, the chorion, and the yolk sac membrane.
The inner cell mass undergoes changes to form three germ layers known as the ectoderm, the mesoderm, and the endoderm. The ectoderm becomes the skin and nervous system, the mesoderm becomes the muscles and other internal organs, and the endoderm becomes the gastrointestinal tract. The embryo is formed at about the fourth week when all the organs of the body have taken shape.
At the age of four weeks, the embryo is about the size of a pea. A primitive heart is beating, the head is defined with rudimentary eyes and ears, and tiny bumps represent arms and legs. The embryo also contains a primitive nervous system, and the head has begun to enlarge. A cartilage skeleton has appeared, and muscles have taken shape.
By the end of eight weeks, the embryo is somewhat human looking. Facial features are evident, and most of the organs are well developed. From this point onward, development consists chiefly of growth and maturation. The embryo is about 1.5 inches in length. Henceforth it is known as a fetus.
Nourishment of the embryo, and then the fetus, is accomplished through the placenta.The maternal and embryonic blood supplies meet at this organ, but the blood does not mix. Instead, diffusion accounts for the passage of gases, nutrients, and waste products across the membranous barriers. The placenta is also an endocrine gland because it secretes estrogen and progesterone to continue to inhibit follicle development and maintain the integrity of the endometrium. As the embryo becomes a fetus, it moves away from the placenta, and a length of tissue called the umbilical cordbecomes its source of attachment to the maternal blood supply.
During the third month, the fetus definitely resembles a human, but the head is relatively large. During the ensuing months, the remainder of the body increases in size proportionally. Cartilage is replaced by bone, and the reproductive organs develop.
During the fourth month, the length of the fetus increases to about 6 inches. The heartbeat can be heard through the mother's abdominal wall, and the fetus moves about. Distinctive movements can be felt at the fifth month, and by the sixth month the fetus weighs almost 2 pounds. By the end of six months, the fetus might be able to survive outside the mother's body, but it would have little fat in its skin so temperature control would be a problem. By the end of the ninth month, the fetus has an average length of about 20 inches and a typical weight of 6 to 8 pounds.