independent/dependent variable in science problem?

As a check of their laboratory protocol for growing mouse embryos in vitro, a group of scientists want to compare the mean mass of 15 embryos grown for two weeks in their lab (see data below) to an average mass value of 75 µg that was obtained in several other published studies that used the same protocol. Is the mass of the mouse embryos grown in this study significantly less than that expected from previously published studies? Assume that all assumptions are met.

What is the independent and dependent variable of this problem?

The answer book said the independent variable was laboratory protocol. Why is this?