Final answer:
The combining word form that means "internal organs" is 'endo-' as in the term 'endoderm,' which is the innermost layer of an embryo that gives rise to many internal organs.
Step-by-step explanation:
The combining word form that means "internal organs" is endo-. The prefix endo- comes from the Greek word endon, meaning 'within' or 'internal'. For instance, the term endoderm refers to the innermost layer of the three primary germ layers in an early embryo, which gives rise to the gastrointestinal tract, the liver, and the pancreas, among other internal organs. In the context of the examples given, the earliest organ to form and begin function within a developing human is the heart, which arises from the mesoderm, not the endoderm or ectoderm. The endoderm, however, does give rise to many internal structures such as the lining of the digestive system and organs like the liver and pancreas.