Gradually, with the emerging field of cytology, researchers began to focus on one type of cell or another, exploring in greater detail their structure and function.

Protoplasm flows and moves, a viscous substance inside cell walls. Apparently constant motion suggested that protoplasm causes cells to move, grow, divide (with help from the nucleus), and interact with each other. Attention shifted to the insides of individual cells, such as white blood cells or amoebae, instead of cell walls or membranes.

In the 1830s, the Germans Matthias Schleiden (with plants) and Theodor Schwann (with animals) declared that plants and animals are all made of cells. They argued that each cell begins with a nucleus, and the cell develops around that nucleus. Cells then join together to make up complex organisms like us.