Final answer:
The texture in the Palestrina's Gloria from the Pope Marcellus Mass, when other voices join the opening phrase, is described as polyphony, indicative of the Renaissance period.
Step-by-step explanation:
When the opening phrase of Palestrina, Gloria, from Pope Marcellus Mass is sung by one person, and then other voices enter and sing together, the texture that best describes the relationship among the voices is called polyphony. This texture is characteristic of the Renaissance period where multiple independent vocal lines are sung simultaneously, creating a rich and complex sound. Polyphony marked a significant departure from the monophonic texture of the earlier Middle Ages and demanded a greater vocal range and blend among ensemble voices.