Which embryonic layer is the innermost of the three primary germ layers?

The innermost of the three primary germ layers is the endoderm. This layer plays a crucial role in the development of several internal structures and organs. It gives rise to the epithelial lining of the gastrointestinal tract, the respiratory system, and other internal organs, such as the liver, pancreas, and thyroid. The endoderm's designation as the innermost layer is significant because it differentiates after the ectoderm and mesoderm layers, which form the outer and middle layers respectively.

Understanding the functions and derivations of the different germ layers is essential in fields such as oncology, where the origin of certain tumors can be traced back to specific germ layers. This knowledge helps in understanding the embryological basis of diseases and their potential treatment strategies.