<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> <P> In 1804, Karl Rudolphi and J.H.F. Link were awarded the prize for "solving the problem of the nature of cells", meaning they were the first to prove that cells had independent cell walls by the Königliche Societät der Wissenschaft (Royal Society of Science), Göttingen . Before, it had been thought that cells shared walls and the fluid passed between them this way . </P> <P> Credit for developing cell theory is usually given to two scientists: Theodor Schwann and Matthias Jakob Schleiden . While Rudolf Virchow contributed to the theory, he is not as credited for his attributions toward it . In 1839, Schleiden suggested that every structural part of a plant was made up of cells or the result of cells . He also suggested that cells were made by a crystallization process either within other cells or from the outside . However, this was not an original idea of Schleiden . He claimed this theory as his own, though Barthelemy Dumortier had stated it years before him . This crystallization process is no longer accepted with modern cell theory . In 1839, Theodor Schwann states that along with plants, animals are composed of cells or the product of cells in their structures . This was a major advancement in the field of biology since little was known about animal structure up to this point compared to plants . From these conclusions about plants and animals, two of the three tenets of cell theory were postulated . </P> <P> 1 . All living organisms are composed of one or more cells </P>

Who proposed the cell theory state the main postulates of this theory