The ectoderm originates in the epiblast, and is formed during gastrulation. Once the mesoderm forms, cells cease to ingress into the primitive streak; the remaining epiblast cells are hereafter called ectoderm.
As organs form, a process called organogenesis, mesoderm interacts with endoderm and ectoderm to give rise to the digestive tract, the heart and skeletal muscles, red blood cells, and the tubules of the kidneys, as well as a type of connective tissue called mesenchyme.
Endoderm forms the epithelium—a type of tissue in which the cells are tightly linked together to form sheets—that lines the primitive gut. From this epithelial lining of the primitive gut, organs like the digestive tract, liver, pancreas, and lungs develop