- Now if you'll remember from near the end of early embryogenesis, you've gone through the
process of gastrulation, and you've formed your
three primary germ layers, and those germ layers were the endoderm, the
mesoderm, and the ectoderm. And the cells in these
germ layers go on to form very specific structures. Endodermal cells are
primarily responsible for forming the gastrointestinal tract. So, I'll draw the gastrointestinal
tract as a tube here, at least to start out with, and we have holes at one
end and at the other. So, the lungs actually develop from the upper gastrointestinal tract, and, of course, the liver as well. And up here in the abdomen
as well we have the pancreas, and then, of course,
that tube goes on to form the esophagus, the stomach, the small intestines,
and the large intestines. Mesoderm tissues, I'll
draw here in purple, form some of the layers of the skin, some of the inner layers. They also form muscles and bones, and that includes the
cardiac muscle as well. And then tucked away down
here in the abdomen and pelvis we have the kidneys and the bladder and our ovaries or testes. And moving on to the ectoderm, as you might suspect, the ectoderm forms the outer layer of skin, and it also forms some skin-related items like sweat glands and
hair, but interestingly, in addition to skin and hair and glands, the ectoderm also forms
our nervous system. So, just a recap. The endoderm forms our gastrointestinal and pulmonary systems,
including the lungs, gastrointestinal tract,
liver, pancreas, et cetera. The mesoderm forms our muscles,
including the heart muscle, our skeletal system, and
our genitourinary tracts. And the ectoderm forms our
skin and related structures, along with our nervous system.