On 7 September 1688 the Dutch microscopist Antoni van Leeuwenhoek finished a letter to the Royal Society in which he demonstrated the circulation of the blood in the external gills of young frog’s larvae, in the tail of older larvae and in the ends of the toes of young and adult frogs (1). Although this letter was not published in the Philosophical Transactions of the Royal Society, as so many of his letters were, Leeuwenhoek himself considered his discovery one of the most important he ever made. He even took care to have the letter published in his own Dutch vernacular before sending it to the learned society in London, this letter being the last to be published separately by him. (2) Leeuwenhoek, whose writings generally were emotionally rather reserved, this time took no pains to hide his enthusiasm: Nay, I saw this movement as clearly as I, or anyone else, could ever imagine the whole propulsion of the blood from the Heart, and the transition of the Arteries (at the place where they join up together) into the Veins. Although I contemplated this sight many times to my exceedingly great pleasure, I did not want to keep it only to myself, but I showed this circulation of the blood to five prominent Gentlemen, who declared to me that they had never yet seen anything of mine that was so worthy of being beheld. (3)

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