 Focusing exclusively on plants, Schleiden asserted that “… every plant … is an aggregate of fully individualized, independent, separate beings, the cells themselves” (1838 [1847: 231–2]). In addition to their walls, Schleiden appealed to the nucleus, which had been identified with improved microscopes by Brown (1833), to identify plant cells. The nucleus figured centrally in Schleiden’s account of cell formation according to which a new cell formed through a process like crystal formation—one type of material is deposited around the nucleolus to form the nucleus and then other material was deposited to form the cell body. The metaphor comparing cell formation to crystal formation played a yet larger role when Schwann confronted the challenge that animal cells differ vastly in their appearance. Schwann argued that they were nonetheless all cells since they all formed through a process analogous to crystal formation: 