Since the FGF receptor being off leads to muscle precursor cells and nerve cord precursor cells, an agent which blocked the receptor would lead to more of those kinds of cells. Since the FGF receptor, when turned on, leads to mesenchyme precursor cells and notochord precursor cells, more would appear if the receptor was always on.
Work Step by Step
Look at the diagram in Fig. 19.7. Note which cells appear whether the the FGF receptor is turned on or off, and extrapolate for the case of the receptor being always on or off to predict what cells would be produced in greater numbers.