Histogenesis

Animals with bilateral symmetry produce a third layer in-between called mesoderm, making them triploblastic.

Germ layers will eventually give rise to all of an animal's or mammal's tissues and organs through a process called organogenesis.

The mesoderm germ layer forms in the embryos of animals and mammals more complex than cnidarians, making them triploblastic.

A theory suggests that this key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals.

Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.

Gastrulation of a diploblast : The formation of germ layers from a (1) blastula to a (2) gastrula . Some of the ectoderm cells (orange) move inward forming the endoderm (red).
The endoderm produces tissue within the lungs , thyroid , and pancreas . The mesoderm aids in the production of cardiac muscle , skeletal muscle , smooth muscle , tissues within the kidneys , and red blood cells . The ectoderm produces tissues within the epidermis and aids in the formation of neurons within the brain, and melanocytes .