Final answer:
The mesoderm and the trophoblast combine to form the placenta, not the germ layers. Of the three germ layers, the mesoderm gives rise to the heart and other structures like muscles, connective tissue, and blood vessels.
Step-by-step explanation:
The Formation of the Placenta
The combination of the mesoderm and the trophoblast contributes to the formation of the placenta. The three germ layers are the ectoderm, mesoderm, and endoderm, which differentiate into various tissues and organs in the embryo. The ectoderm gives rise to features such as the nervous system and skin, while the endoderm forms internal linings of the digestive and respiratory tracts. Specifically, the mesoderm is the layer that ultimately gives rise to the heart, along with muscles, connective tissue, blood vessels, and kidneys. The trophoblast is part of the outer layer of the blastula and plays a critical role in the development of the placenta but is not itself a germ layer.