In an experiment to determine whether M phase cells contain a regulatory molecule that initiates mitosis, cytoplasm from M phase cells and interphase cells was injected separately into interphase cells. Why did researchers inject cytoplasm from interphase cells

As a negative guide, to demonstrate that perhaps the injection itself has no impact on the cell cycle.

A successful experiment has handled the same control group as the experimental group with the exception of the variable being examined, in this case the cytoplasm M-phase.