|The Germinal Period of
The one-celled zygote created at conception divides by mitosis into 2 genetically identical cells within 12 to 15 hours... and then another division to make 4 cells within the next 12-15 hours... and so forth. By the third day following conception the cells have divided to form a small ball that is known as a blastocyst. The blastocyst enters the uterus and begins to implant in the blood rich lining of the uterus.
Implantation occurs when the blastocyst sinks into the endometrium. Through the action of hormones produced by the blastocyst, an intensive chemical exchange occurs between the blastocyst and the mother's tissue to signal its presence in the uterus. This exchange temporarily impairs the mother's immune system to prevent antibodies from attacking the blastocyst as foreign matter.
During the second week after conception, the inner cell mass of the blastocyst structure separates into three distinct layers that may be seen under a microscope. The outer layer is known as the ectoderm, which will later form the skin, hair, sweat glands, tooth enamel, salivary glands, and all the nervous tissue, including the brain. The middle layer, or mesoderm, will form the muscles, bones, blood, circulatory system, teeth, connective tissues and kidneys. The inner layer, the endoderm, will form most of the internal organs (stomach, intestines, liver, lungs, heart, and so on).
These three layers are differentiated by the third week following conception. They will continue to proliferate into the various organ system tissues during the embryonic period of prenatal development.