Organogenesis:
Organogenesis is the formation of organs. As development continues, cells begin to
differentiate, taking on different sizes and shapes. Differentiation leads to the formation of specialized cells that can perform specific functions. These cells become organized into
tissues which, in turn, form
organs.
The differentiation of unspecialized cells (
stem cells) into tissues and then organs marks the transition from a gastrula to an
embryo. Particularly in mammals, when the organ systems are more or less completely formed, the embryo is referred to as a
fetus.