Virchow lived from 1921-1902, and is considered the founder of modern pathology. He discovered fibrinogen, leukocytosis, leukemia, and myelin, worked out the conditions that predispose to thrombosis, explained and named pulmonary embolization, and refuted the current fad theory of disease -- the humoralism of "crases" and "dyscrasias" which prohibited surgery for localized lesions. He went on to establish the true nature of pus, necrotic cells, sarcomas, red infarcts, amyloid, metastatic calcification, erythrophagocytosis, uterine fibromyomas, brown induration of the lung, malaria pigment, trichinosis, echinococcal cysts, uric acid nephropathy, and psammoma bodies. He discovered and named neuroglia, gliomas, and giant cells. He coined the words "hyperplasia" and "ischemia", and named many of our common tumors. He wrote the first descriptions of the systemic fungal diseases, and discovered the amino acids leucine and tyrosine. He established that all cells today arise from pre-existing cells.
(from http://www.pathguy.com/lectures/virchow.htm)

Cells had been known to exist since the 1600s, when Robert Hooke looked at thin sections of cork and other plant tissues in 1665, introduced the term cell because the cellulose walls of dead cork cells reminded him of the blocks of cells occupied by monks. Even after the publication in 1672 of excellent pictures of plant tissues, no significance was attached to the contents within the cell walls. The magnifying powers of the microscope and the inadequacy of techniques for preparing cells for observation precluded a study of the intimate details of the cell contents. The inspired amateur of early microscopy Antonie van Leeuwenhoek, beginning in 1673, discovered blood cells, spermatozoa, and a lively world of "animalcules." A new world of unicellular organisms was opened up. Such discoveries extended the known variety of living things but did not bring insight into their basic uniformity. Moreover, when Leeuwenhoek observed the swarming of his animalcules but failed to observe their division, he could only reinforce the idea that they arose spontaneously.

Cell theory was not formulated for nearly 200 years after the introduction of microscopy. Explanations for this delay range from the poor quality of the microscopes to the persistence of ancient ideas concerning the definition of a fundamental living unit. Many observations of cells were made, but apparently none of the observers was able to assert forcefully that cells were the units of biologic structure and function.

Three critical discoveries made during the 1830s, when improved microscopes with suitable lenses, higher powers of magnification without aberration, and more satisfactory illumination became available, were decisive events in the early development of cell theory. First, the nucleus was observed by Robert Brown in 1833 as a constant component of plant cells. Next, nuclei were also observed and recognized as such in some animal cells. Finally, a living substance called protoplasm was recognized within cells, its vitality made evident by its active streaming, or flowing, movements, especially in plant cells. After these three discoveries, cells, previously considered as mere pores in plant tissue, could no longer be thought of as empty, because they contained living material.

Two German biologists, Theodore Schwann and Matthias Schleiden, clearly stated in 1839 that cells are the "elementary particles of organisms" in both plants and animals and recognized that some organisms are unicellular and others multicellular. This statement was made in Schwann's Mikroskopische Untersuchungen über die Übereinstimmung in der Struktur und dem Wachstume der Tiere und Pflanzen (1839; Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants). Schleiden's contributions on plants were acknowledged by Schwann as the basis for his comparison of animal and plant structure.

Schleiden and Schwann's descriptive statements concerning the cellular basis of biologic structure are straightforward and acceptable to modern thought. They recognized the common features of cells to be membrane, nucleus, and cell body and described them in comparisons of various animal and plant tissues. A statement by Schleiden pointed toward the future direction of cell studies:

Each cell leads a double life: an independent one, pertaining to its own development alone; and another incidental, insofar as it has become an integral part of a plant. It is, however, easy to perceive that the vital process of the individual cells must form the first, absolutely indispensable fundamental basis, both as regards vegetable physiology and comparative physiology in general. . . .

(this is from http://www.britannica.com/bcom/eb/article/8/0,5716,108768+1,00.html)

And since Virchow graduated from medical school in 1843, and started his research, we can assume that the cell theory had been postulated by that time. However he has contributed enormously to our knowledge of diseases.

the other websites I used in answering this question were:
http://www.britannica.com/bcom/eb/article/7/0,5716,77447+1+75460,00.html
http://www.britannica.com/bcom/eb/article /1/0,5716,119731+33+110576,00.html?query=cell%20theory

When I first started looking for the answer to this question, I went to www.google.com and did an advanced search, since when I entered "Rudolph Virchow" as the simple search term, I got a whole lot of websites in German, which I don't read. However, the advanced search allows you to specify the language of the responses--very helpful. Encyclopedia Britannica also has a wonderful website, which I believe you can search for free if you click on the 'school' button.

One of the biggest problems with searching the web is that ability to rapidly plagiarize stuff. It took me only a couple seconds to cut and paste this response together. As long as you teach your students about the importance of citing their facts, and you're diligent about checking it out, it's not too big a problem (lots of work for you though...)

I like the format of your question since you can't just look up Virchow and get the whole answer, I had to search through several sites, as well as use the info I already know about the history of biology (which is pathetically little).

Anyway....I hope this was helpful. Please let me know if I can help you with anything else!

Best,
Sarah Tegen