The Germinal Stage | The germinal stage begins with conception, when the sperm and egg cell unite in one of the two fallopian tubes. The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete. Cell division begins approximately 24 to 36 hours after conception. Cell division continues at a rapid rate and the cells then develop into what is known as a blastocyst. The blastocyst is made up of three laters: the ectoderm (which will become the skin and nervous system), the endoderm (which will become the digestive and respiratory systems), and the mesoderm (which will become the muscle and skeletal systems). Finally, the blastocyst arrives at the uterus and attached to the uterine wall, a process known as implantation.
The Embryonic Stage | The mass of cells is now know as and embryo. The embryonic stage begins after implantation and continues until cell differentiation has been mostly completed. Structures important to the support of the embryo develop, including the placenta and umbilical cord. During this time, cells begin to differentiate into the various body systems. The basic outlines of the organ, body, and nervous systems are established. By the end of the embryonic stage, the beginnings of features such as fingers, eyes, mouth, and ears become visible.
The Fetal Stage | Once cell differentiation is mostly complete, the embryo enters the next stage and becomes known as a fetus. The early body systems and structures established in the embryonic stage continue to develop. The neural tube develops into the brain and spinal cord and neurons form. Sex organs begin to appear during the third month of gestation. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the later stages of pregnancy.
Resource by By Kendra Cherry, About.com Guide