Gastrulation
The mesoderm forms the somites, the notochord, and the mesenchyme, which give rise to the muscles, circulatory and excretory systems of the body. This key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals. The formation of a mesoderm led to the development of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.