What was Leeuwenhoek talking about when he used the term " animalcules"?

1590s–1720s). Using single-lensed microscopes of his own design, van Leeuwenhoek was the first to experiment with microbes, which he originally referred to as dierkens, diertgens or diertjes (Dutch for "small animals" [translated into English as animalcules, from Latin animalculum = "tiny animal"]).
Parents: Philips Antonisz van Leeuwenhoek, Margaretha Bel van den Berch
Died: 26 August 1723, Delft
Born: 24 October 1632, Delft
Nationality: Dutch Republic, Netherlands

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