Cells each have a cytoplasm and nucleus, but they do not all look or work in exactly the same ways. To get at how differences arise and what they mean, Wilson and others carried out detailed studies of cell lineage, that is study of the lineages of cells, how one cell divided into two and so on. Wilson, Edward Grant Conklin, and others at the Marine Biological Laboratory in Woods Hole, Massachusetts, spent considerable energy collecting specimens from different species, taking them into the lab and observing every stage as each cell divided, one by one.
This work involved observing living cells, and also preserving, fixing, staining, and in short killing them in order to observe what was going on inside. They could see the changes in shape and structure, and they could see the way the chromosomes and other parts behaved during cell division and during differentiation. They could see that it depends on where each cell is located within the organism what shape it takes and how it divides in the next step. But they could only observe for so long in the organism’s developmental process; eventually there were too many cells, and the complexity of the whole organism made it impossible with this method to continue observing the individual cellular parts.