Theory of Cells as Basic Units of Life

Better microscopes allowed observations and drawings that led to a Cell Theory for all of life, though the idea was not accepted immediately.

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In the 1830s, the Germans Matthias Schleiden (with plants) and Theodor Schwann (with animals) declared that plants and animals are all made of cells. They argued that each cell begins with a nucleus, and the cell develops around that nucleus. Cells then join together to make up complex organisms like us.

They argued about whether cells arise by crystallization from surrounding material, through spontaneous generation, by cell division, or some other process.

Illustrations of animal cells HoverTouch to magnify
Animal cells

Schwann 18381

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Illustrations of plant cells HoverTouch to magnify
Plant cells

Schleiden 18382

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In the 1850s, the Polish-German Robert Remak showed that cells arise by division of other cells, then German pathologist Rudolf Virchow popularized the idea. Though science does not progress through instant agreement with new ideas, this assumption became part of what was accepted by 1900 as the cell theory. The ability to observe cell division and represent it in images eventually persuaded others.

Illustrations of plant cells dividing HoverTouch to magnify
Cells dividing

Remak 18553

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Illustration of cells dividing by Virchow HoverTouch to magnify
Cells dividing

Virchow 18584

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The cell theory established that cells are the fundamental units of living organisms, which arise through cell division, and that cells each have a nucleus and also material called protoplasm. But what is protoplasm and what is its role in life?
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  1. Schwann, Theodor. Mikroskopische Untersuchungen. 1838鈥39. (Translated by Henry Smith as 鈥淢icroscopical Researches,鈥 in Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals. London: The Sydenham Society, 1847.) Plate 1.
  2. Schleiden, Mattias Jacob. 1838. "Beitr盲ge 眉ber Phytogenesis." M眉ller's Archiv f眉r Anatomie and Physiologie (1838): 137鈥176. (Translated by Henry Smith as "Contributions to Phytogenesis," in Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals. London: The Sydenham Society, 1847: 229-63.) Plate 1, Figures 1-8 and 22-24. 
  3. Remak, Robert. Untersuchungen 眉ber die Entwicklung der Wirbelthiere. Berlin: Reimer, 1855. Table 3, Figure 35, A and B.
  4. Virchow, Rudolf. Die Cellularpathologie. 4th Edition. Berlin: Hirschwald, 1871. Page 18, Figure 2.