The microscope was the key invention for discovering and describing cells.
The cell is still being discovered. It didn’t stop
with discovery of nuclei,
or the Golgi apparatus, or the flagella, cilia, or pili,
or the ribosomes, mitochondria, centrioles, chloroplasts, peroxisomes, or lysosomes,
or the endoplasmic reticula.
We are still working out how these things work.
See also in The book of science:
Readings on wikipedia: