Mesoderm, ectoderm and endoderm tissues in animal embryos grow into every organ the animal needs at birth, according to The Embryo Project Encyclopedia at Arizona State University. Collectively, scientists refer to these three layers of tissue as germ layers, and they form early in an embryo's life through a process called gastrulation.
The ectoderm forms outer parts of the body such as skin, hair, eyes, mammary glands and the spinal cord. When a part of the ectoderm folds inward, the neural tube grows in the embryo to create the central nervous system, parts of the face and the brain, according to Arizona State University.
The endoderm creates major organs such as the liver, pancreas, lungs and thyroid. The Biology Project from the University of Arizona explains that this germ layer also creates the mucosa lining of the digestive tract, tonsils and the inner layer of the lower urinary tract. The mesoderm grows into skeletal muscles, bones, connective tissues and the heart and forms the kidneys and the dermis of the skin.
LifeMap Sciences reveals that the three germ layers start to separate into distinct sections within the first three weeks of embryonic development. A small, spherical mass of cells rapidly forms the ectoderm, endoderm and mesoderm between day nine and day 21 of a human embryo's existence.