The endoderm is the innermost germ layer, which makes it easy to remember the body parts it gives rise to: stuff inside our bodies, specifically, spaces and cavitations like the gut tube and body cavities that house our internal organs.
During week 4 of development, the embryo folds in two directions.
In the longitudinal plane, the embryo folds slightly at the cranial and caudal ends, so it looks less like a pancake and more like a little shrimp.
This is the very beginning of its curling into the fetal position.
This folding shapes part of the yolk sac into a gut tube, with the remainder of the yolk sac remaining connected not at the cranial or caudal end, but just in the middle.
Now let’s switch to the transverse plane, looking at a spot near the middle of our embryo where you can see the yolk sac.
The visceral layer of mesoderm follows the endoderm and forms the gut tube.
More specifically, the endoderm becomes the epithelial cell lining of the gastrointestinal tract while the visceral layer of mesoderm becomes the muscular wall.
The mesoderm also gives rise to serous membranes that become the visceral and parietal pleura.
Interestingly, the mesentery was only recently officially classified as a body organ!
Meanwhile, the ectoderm and parietal layer of mesoderm fold around the dorsal side of the embryo, with the two sides meeting up and seamlessly coming together in the midline to form the anterior body wall.
Well, we say “seamlessly”, but actually, the yolk sac still pouches out a little.
Once that happens, we end up with the hollow gut tube inside the embryo’s body, so in a sense embryonic folding creates a tube inside a tube.