The creation of new life is an amazing event, but one that does not happen overnight. That cute little bundle of joy in the arms of his parents went through many different developmental stages. Prior to entering the world, the baby is known as a fetus. The fetus stage begins approximately eight weeks after fertilization, or during the 10th week of pregnancy according to the Merck Manual. During the first eight weeks the fetus went through several early embryo developmental stages.
Zygote
To create a baby the ovum, or egg, of a woman must be fertilized by the sperm of a man. This process alone is not an easy one, since out of hundreds of millions of sperm it is lucky for one to locate and penetrate the egg. Once the sperm does penetrate the egg, the egg is considered fertilized.
Once the nucleus of the egg and the nucleus of the sperm fuse, which occurs within 11 hours of fertilization according to The Visible Embryo, it is referred to as a zygote. The zygote now contains 23 chromosomes from the egg and 23 chromosomes from the sperm.
Fertilization typically occurs in the fallopian tube. Once the zygote has formed, it slowly makes its way through the fallopian tube to the uterus. While traveling, the zygote begins to divide. The one cell divides into two cells, called blastomeres, which each divide into two more and so on.
Morula
When the zygote consists of 16 cells, it is referred to as a morula. By this time, usually between three and five days after fertilization, it enters the uterus. The cells continue to divide, forming a cavity known as a blastocele in the center of the morula.
Blastocyst
Once the cavity, or blastocele, is formed the morula is referred to as a blastocyst. At this point there are two different cell types forming. The embryoblast cells are those cells forming on the inside of the blastocele and the trophoblast cells are those forming on the outside of the blastocele. The trophoblast cells secrete enzymes that allow the blastocyst to implant in the wall of the uterus, which usually occurs around day nine or 10.
The trophoblast cells will develop into the placenta to provide nutrients to the developing embryo. The embryoblast cells become the embryo and the amniotic sac, which becomes filled with fluid in which the embryo will float.
Embryo
Approximately 13 to 14 days after conception, a line of cells known as the primitive streak begins to appear. These cells create bilateral symmetry, which means that spontaneous division is no longer possible. At this point the blastocyst becomes an embryo. Although the embryo is still a mass of undifferentiated cells, differentiation and organ formation soon begins as the embryo undergoes rapid developmental changes through the next several weeks.
