 When Hooke introduced the term cell in Micrographia in 1665 to describe the microscopic structure of cork that he observed using a compound microscope  (Figure 2A),  he was comparing cells to the polygonal cells of beeswax and the small rooms occupied by monks in a monastery. (Hooke 1665: 116, also commented on the fluid content of cells, labeling them “succus nutritus, or appropriate juices of vegetables”. As discussed below, the fluid content of cells took on new importance with the advent of the protoplasm theories in the mid-nineteenth century.) Although researchers who focused on plants tended to find Hooke’s metaphor appropriate, since plant cells have clearly observable walls, others, beginning with van Leeuwenhoek, who examined spermatozoa  (Figure 2B),  animal tissues, and bacteria, tended to adopt other terms such as corpuscle or globule. 