<P> Optical microscopes can focus on objects the size of a wavelength or larger, giving restrictions still to advancement in discoveries with objects smaller than the wavelengths of visible light . Later in the 1920s, the electron microscope was developed, making it possible to view objects that are smaller than optical wavelengths, once again, changing the possibilities in science . </P> <P> The cell was first discovered by Robert Hooke in 1665, which can be found to be described in his book Micrographia . In this book, he gave 60' observations' in detail of various objects under a coarse, compound microscope . One observation was from very thin slices of bottle cork . Hooke discovered a multitude of tiny pores that he named "cells". This came from the Latin word Cella, meaning' a small room' like monks lived in and also Cellulae, which meant the six sided cell of a honeycomb . However, Hooke did not know their real structure or function . What Hooke had thought were cells, were actually empty cell walls of plant tissues . With microscopes during this time having a low magnification, Hooke was unable to see that there were other internal components to the cells he was observing . Therefore, he did not think the "cellulae" were alive . His cell observations gave no indication of the nucleus and other organelles found in most living cells . In Micrographia, Hooke also observed mould, bluish in color, found on leather . After studying it under his microscope, he was unable to observe "seeds" that would have indicated how the mould was multiplying in quantity . This led to Hooke suggesting that spontaneous generation, from either natural or artificial heat, was the cause . Since this was an old Aristotelian theory still accepted at the time, others did not reject it and was not disproved until Leeuwenhoek later discovers generation is achieved otherwise . </P> <P> Anton van Leeuwenhoek is another scientist who saw these cells soon after Hooke did . He made use of a microscope containing improved lenses that could magnify objects almost 300-fold, or 270x . Under these microscopes, Leeuwenhoek found motile objects . In a letter to The Royal Society on October 9, 1676, he states that motility is a quality of life therefore these were living organisms . Over time, he wrote many more papers in which described many specific forms of microorganisms . Leeuwenhoek named these "animalcules," which included protozoa and other unicellular organisms, like bacteria . Though he did not have much formal education, he was able to identify the first accurate description of red blood cells and discovered bacteria after gaining interest in the sense of taste that resulted in Leeuwenhoek to observe the tongue of an ox, then leading him to study "pepper water" in 1676 . He also found for the first time the sperm cells of animals and humans . Once discovering these types of cells, Leeuwenhoek saw that the fertilization process requires the sperm cell to enter the egg cell . This put an end to the previous theory of spontaneous generation . After reading letters by Leeuwenhoek, Hooke was the first to confirm his observations that were thought to be unlikely by other contemporaries . </P> <P> The cells in animal tissues were observed after plants were because the tissues were so fragile and susceptible to tearing, it was difficult for such thin slices to be prepared for studying . Biologists believed that there was a fundamental unit to life, but were unsure what this was . It would not be until over a hundred years later that this fundamental unit was connected to cellular structure and existence of cells in animals or plants . This conclusion was not made until Henri Dutrochet . Besides stating "the cell is the fundamental element of organization", Dutrochet also claimed that cells were not just a structural unit, but also a physiological unit . </P>

Who concluded that all cells come from preexisting cells