 So the ability to look at small things and magnify them with microscopes led to the concept of the cell as the fundamental unit of life. So biologists define all living organisms as based around a single cell and larger organisms can have many of these cells. So Robert Hooke described the cell theory or coined the term cell from his drawing of a very finely sliced piece of cork and it looked like a sort of set of rooms of an apartment block, multi-storey apartment block. But over time it required other information to establish what the cell actually did. Now, Lewin Hooke's simpler microscope allowed him to see contents inside the cell and you could actually see the nucleus, although Lewin Hooke didn't describe that and it was quite a while later that the compound microscope became sufficiently well built that Brown actually described the nucleus as this body contained within cells. Subsequently there were two German biologists, a plant biologist and a zoologist who independently came up with the idea that this nucleus was very important in controlling cell division. So at that stage the role of the nucleus was not understood but it was seen that this little black spot or body like a balloon sitting inside the cell was crucial to the functioning of the cell. Now the reason that's important of course is how do animals come about. Earlier people had thought it arrived from the miasma or just spontaneous creation and gradually people realised no you had to have a preceding living cell in order to generate the next cell.