Study of cells. Its earliest phase began with Robert Hooke's microscopic investigations of cork in 1665, during which he introduced the term cell to describe dead cork cells. Mathias Jacob Schleiden (in 1838) and Theodor Schwann (1839) were among the first to state clearly that cells are the fundamental units of both plants and animals. This pronouncement (the cell theory) was confirmed and elaborated by a series of discoveries and interpretations. In 1892 Oscar Hertwig (1849–1922) suggested that processes at the organism's level are reflections of cellular processes, thus establishing cytology as a separate branch of biology. See alsophysiology.