The developing stage is a term used to refer to the period of time in which an EMBRYO is growing and maturing. This stage typically begins after fertilization and ends when the EMBRYO is fully developed and ready for birth. During this stage, the EMBRYO will go through several changes, including cell division, organ formation, and the development of the nervous system. The EMBRYO will also begin to move and respond to stimuli. This stage is critical for the successful development of the fetus and is essential for a healthy pregnancy.