A muscle is a very large number of fibers close together, containing two different substances myosin and actomyosin.

He discovered that myosin is only needed for the first and final stages of the amoeba's development and that, in all stages between, myosin is like a candy bar after lunch — nice but not needed.

One that churns out actin and myosin, which link up to form units that can expand and contract, will become a muscle cell. And so on.