MICROSCOPES AND THE STUDY OF LIFE AND CELL. It was soon discovered that not only were large organisms made up of huge numbers of tiny cells- multicellular organisms, but tiny organisms existed made up of as few as a single cell unicellular organisms or single-celled organism, and it was found that in the unicellular realm, there was a basic either-or difference in organisms: theirs might be a cell with a nucleus, like the cells found in multicellular organisms eukaryotes or a cell without a nucleus, assumed to be simpler and primitive prokaryotes and not found in multicelled forms. The unicellular level, the distinctions between animals and plants were less reliable. It is not unusual to find unicellular organisms that are mobile and food-gathering like animals but which are also green and photosynthetic.

Were these animals or plants? As we have seen in the multi-Kingdom system, the definition difficulties were addressed by giving the unicellular forms their own Kingdoms, where they could continue to multi-task without upsetting humans and their labeling compulsions. In 1838, Cell theory of the living things was proposed by the two scientists namely Matthais Schleiden worked with plants and may have come up with most of the hypothesis and Theodore Schwann extended to animals. Cell Theory for today says that Everything alive is made up of at least one cell. The smallest organisms are single-celled, or unicellular. There is a range of complexity in cells covered later, but the size range in cells also exists, with smaller cells generally being simpler.

Larger organisms are made up of several to trillions of cells, and are multicellular. In these organisms, the cells work together, with different cells doing different jobs to contribute to the function of the whole, each cell being dependent upon the contributions of the others. As often happens in biology, this is not totally a situation of either-or; between unicellular and multicellular are organisms that are unicellular and colonial, with cells that technically could live independent of each other, but which band together and split up functions similar to the way it is done in multicellular organisms. Cells can only be made from existing, related cells. For a very long time, people believed in spontaneous generation. At the time of Schleiden and Schwann, and even well after, biologists thought that cells could be generated from such nonliving material as boiled chicken broth.

The Cell Theory said that no, you can only get living cells from cells that are already there. It was an eventual addition that cells are only made by related cells - after germs were discovered; it was thought for some time that our body cells spontaneously generated the bacteria that carried diseases between people, eukaryotes producing prokaryotes. We now know that the bacteria use us as a source of nutrition and as their own little private ecosystems to make lots of offspring bacteria that we are nice enough to pass on to other people. Cell is also considered as the smallest single thing alive. Since their microscopes limited what they could see below a certain size. It was almost another hundred years before viruses were discovered, and the bits and pieces inside cells that they could see seemed just that, contributory fragments that would be nothing by themselves.

Today, viruses are controversial because they break so many rules, and some cell bits are much more cellular than anyone suspected they would be. Cells of all living things are more alike than different. This was not a part of the original theory, although the two researchers seemed to realize over time that what one discovered in plant cells the other was seeing in animal cells. It was still an era where terms were based more on how things differed than how they were the same, and for over a century any hypotheses that tried to apply rules across the whole of biology were controversial at least partly because of that.