Embryo

An embryo is the earliest stage of prenatal development, occurring between fertilization and birth. During this period, a single-celled egg divides into ever-more-complex cell clusters, eventually forming all of the tissues, organs, and systems that make up a newborn baby. Embryo development is a complex process, and scientists are still working to understand the molecular and genetic mechanisms behind it. In mammals, the embryo undergoes three distinct stages of development. In the first stage, the blastocyst, the fertilized egg implants in the uterine wall and begins to rapidly divide. The blastocyst is made up of two layers of cells, the trophoblast and the inner cell mass, which will eventually form the fetus. During the second stage, the embryo undergoes gastrulation, in which the cells migrate and begin to form a three-layered structure called the gastrula. The outer layer forms the ectoderm, which will become the skin and nervous system, the middle layer forms the mesoderm, which will become the muscles and organs, and the inner layer forms the endoderm, which will become the digestive and respiratory systems. In the third stage, the embryo undergoes organogenesis.