Final answer:
The third part of the cell theory, stating that new cells arise from existing cells, was added by Rudolf Virchow in 1858, about two centuries after Robert Hooke first observed cells. This finalized the fundamental elements of cell theory that still stand today in biology.
Step-by-step explanation:
The third part of the cell theory was added in the 1850s, specifically in 1858, by Rudolf Virchow who proposed that all cells arise only from other cells. The time interval between the emergence of this component of cell theory and the initial discovery of cells was approximately two centuries. Cells were first observed in the 1660s by Robert Hooke, signaling the dawn of cellular biology, but the comprehensive cell theory wasn't established until the 19th century following the contributions of numerous scientists.
Robert Hooke coined the term "cell" when he first observed the tiny structures in cork. Anton van Leeuwenhoek later described tiny organisms he called animalcules, which are now known to be single-celled organisms. The three main parts of cell theory are: all organisms are made of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.
Common parts to all cells include the plasma membrane, cytoplasm, ribosomes, and genetic material (DNA). Ribosomes are the cell structures where proteins are made, and DNA's crucial role involves carrying the genetic information necessary for the organization, functioning, and reproduction of cells. For studying the structures within cells, a transmission electron microscope would be ideal due to its high resolution.