Final answer:
The endoderm, the innermost germ layer of an embryo, gives rise to the cells that form the gastrointestinal tract, liver, pancreas, and the lining of the respiratory system.
Step-by-step explanation:
It is responsible for forming the mature cells of multiple internal organs and glands. Specifically, the endoderm will develop into the epithelial lining of the gastrointestinal tract, which includes the stomach and intestines, plus vital organs such as the liver and pancreas. Additionally, it gives rise to the lining of the lower respiratory system, including the trachea and lungs, and other internal structures like the thyroid gland.