Who plays the contrasting sections in a ritornello

During a concerto grosso, the first and last movements would often be in ritornello form. Ritornello form is based on the alternation between the tutti and soloist sections of the piece. In ritornello form, the tutti opens the piece with a main theme called the ritornello, which means "refrain." This theme, which is always played by the tutti, returns throughout the piece in different keys and in pieces; it usually does not return in its entirety until the end of the movement. 

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