Cells were first observed in the 1650s with the invention of the microscope. Early scientists like Leeuwenhoek and Hooke made early observations of cells but it was not until the 19th century that the cell theory was developed. The cell theory states that all living things are made of cells, cells are the basic functional units of life, and new cells are produced from existing cells. Many scientists through the 1800s performed experiments disproving the theory of spontaneous generation and showing that cells only come from other cells, culminating in Pasteur's experiments with swan neck flasks. The cell contains organelles that allow it to carry out life functions like the nucleus, mitochondria, chloroplasts, and cell membrane.