Embryology is the
science of the early development of organisms , between the one-cell stage (generally, but not always, the
zygote ) and the beginning of free living. Embryology therefore deals with the various steps necessary for the correct and complete formation of the
body of a living organism. Born as a descriptive science, modern embryology is involved in studying the molecular mechanisms that underlie the development of the
embryo ; see also
developmental biology.