Every organ in your body traces back to one of three cell layers
Skin and brain, muscle and bone, gut and lungs: every tissue in an adult human can be traced to one of three sheets of cells that formed when you were an early embryo. The process, called gastrulation, folds a hollow ball of cells into a cup and quietly assigns each future organ its origin.
An embryo is the first stage of a many-celled organism's life. In sexual reproduction it begins when sperm fuses with egg to form a single cell, the zygote. What follows is cleavage, a burst of rapid divisions in which the cell count soars while the embryo as a whole stays the same size, so each cell becomes steadily smaller. The cells, called blastomeres, form a solid ball known as a morula, which then takes in fluid to make a cavity. The result is a blastula, or in mammals a blastocyst.
Depending on the species, a blastula may be a cluster perched on yolk or a hollow sphere. Inside the cells, RNAs and proteins are already directing gene activity, deciding cell fates and establishing polarity. Before it implants, the mammalian embryo is sometimes called a pre-implantation embryo, or pre-embryo, a term used in debates about embryonic stem cells. The blastocyst hatches and then implants in the lining of the womb.
Gastrulation comes next. Cnidarians form just two layers; most others, from flatworms to humans, form three. The outer ectoderm becomes skin and the nervous system. The middle mesoderm builds blood vessels, muscle, bone and connective tissue. The inner endoderm forms the digestive organs and the linings of the gut and airways. As the layers migrate, the round embryo folds inward. Neurulation then lays down the nervous system, and organogenesis builds the rest.
In humans the word embryo applies until the ninth week after conception, after which the developing individual is called a fetus; in other organisms it can mean any stage before birth or hatching. The word entered English in the mid-14th century, from a Greek term for young one, built from roots meaning in and to swell.
Source: Embryo
