Question

which discovery was the earliest step in the scientific development of the modern cell theory?
aristotle discussed the idea of spontaneous generation.
flemming used dyes to stain cells so they could be observed more clearly.
hooke and von leeuwenhoek observed cells and tiny organisms with microscopes.
schleiden and schwann published papers stating that plants and animals were made of cells.

Explanation:

Aristotle's idea of spontaneous generation is not directly related to cell - theory development. Flemming's work on staining cells came later. Schleiden and Schwann's work was building on earlier observations. Hooke and van Leeuwenhoek's microscopic observations of cells and tiny organisms were early steps in the development of cell theory. Their work laid the ground - work for understanding the existence of cells.

Answer:

Hooke and von Leeuwenhoek observed cells and tiny organisms with microscopes.