Swammerdam, Jan (yän väˈmərdäm) [key], 1637–80, Dutch naturalist. He was a pioneer in the use of the microscope. Before he turned to religious contemplation his chief interest was the study of invertebrates. He investigated the life histories of frogs and of numerous insects, which he classified on the basis of their metamorphic development. He also made valuable observations on human anatomy and was probably the first to detect red blood cells (1658). A composite collection of his descriptions and of his accurate and exquisitely executed drawings was published posthumously (2 vol., 1737–38) and appeared in English as The Book of Nature (1758). He was an early and influential proponent of the theory of evolution, in opposition to the current belief in spontaneous generation.
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