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The study of cells – cell biology – began in 1660, when English physicist Robert Hooke melted strands of spun glass to create lenses that he focused on bee stingers, fish scales, fly legs, feathers, and any type of insect he could hold still. When he looked at cork, which is the bark from a type of oak tree, it appeared to be divided into little boxes, which were remnants of cells that were once alive. Hooke called these units "cells" because they looked like the cubicles (cellae) where monks studied and prayed. Although Hooke did not realize the significance of his observation, he was the first person to see the outlines of cells. In 1673, Antony van Leeuwenhoek of Holland improved lenses further. He used only a single lens, but due to its quality, it was more effective at magnifying and produced a clearer image than most two-lens microscopes then available. One of his first objects of study was tartar scraped from his own teeth, and he observed that it contained many very small animalcules (microscopic organisms). Over the next few years, Leeuwenhoek built more than 500 microscopes that opened a vast new world to the human eye and mind. He viewed bacteria and other microorganisms – life that people had not known existed. However, he failed to see the single-celled "animalcules" reproduce, and therefore he perpetuated the popular idea at the time that life arises from the nonliving or from nothing. Nevertheless, he described with remarkable accuracy microorganisms and microscopic parts of larger organisms, including human red blood cells. Despite the accumulation of microscopists' drawings of cells made during the seventeenth and eighteenth centuries, the cell theory – the idea that the cell is the fundamental unit of all life – did not emerge until the nineteenth century. Historians attribute the delay to poor technology – for example, crude microscopes and a lack of procedures to preserve and study living cells without damaging them. Neither the evidence itself nor early interpretations of it suggested that all organisms were composed of cells. Hooke had not observed actual cells but rather what they had left behind: the cell walls. Leeuwenhoek made important observations, but he did not methodically describe or categorize the structures that cells had in common. In the nineteenth century, more powerful microscopes, with better magnification and illumination, revealed details of life at the subcellular level. In the early 1830s, Scottish surgeon Robert Brown noted a roughly circular structure in cells from orchid plants. Finding the structure in every orchid cell, he then identified it in all cells from a variety of other organisms. He named it the "nucleus," – a term that had remained in use. Brown memorialized the importance of the structure he discovered, but today we know the nucleus houses DNA for complex cells. The cell theory finally emerged in 1839 when German biologists Matthias J. Schleiden and Theodore Schwann made careful comparisons of plants and animals. Schleiden first noted that cells were the basic units of plants, and then Schwann compared animal cells to plant cells. After observing many different plant and animal cells, they concluded that cells were "elementary particles of organisms, the unit of structure and function." Schleiden and Schwann described the components of the cell as a cell body and nucleus contained within a surrounding membrane. Schleiden called a cell a "peculiar little organism" and realized that a cell can be a living entity on its own; but the new theory also recognized that in large plants and animals, cells are part of a larger living organism. Many cell biologists extended Schleiden and Schwann's observations and ideas. German physiologist Rudolph Virchow added the important corollary in 1855 that all cells come from preexisting cells, contradicting the still-popular idea that life can arise from the nonliving or from nothingness. Virchow's statement also challenged the popular concept that cells develop on their own from the inside out, the nucleus forming a cell body around itself, and then the cell body growing a cell membrane. Virchow's observation set the stage for descriptions of cell division in the 1870s and 1880s. Virchow was ahead of his time because he hypothesized that abnormal cells cause diseases that affect the whole body.