In triploblastic organisms,
the three germ layers are called endoderm, Ectoderm, and mesoderm.
For example, in neurogenesis, a subpopulation of Ectoderm cells is set aside to become the brain,
spinal cord, and peripheral nerves.
Ectoderm: it is the outermost layer,
from which the nervous system, the mouth, the skin and its outermost part of the body will be formed.
Around day 15 of development, the cells that will become the heart exist in two horseshoe shaped bands of the middle tissue layer(mesoderm),
and some cells migrate from a portion of the outer layer(Ectoderm), the neural crest,
which is the source of a variety of cells found throughout the body.