This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CVS Embryology, Subject name: Anatomy
[{'content': 'Which of the three primary germ layers forms the histologically definitive endocardium of the adult hea? \nSelect and explain. \nA. Ectoderm, B. Endoderm, C. Mesoderm, D. Epiblast', 'role': 'user'}, {'content': 'Answer: C \nExplanation: C. The entire cardiovascular system is of mesodermal origin.', 'role': 'assistant'}]