Letter, from Antoni van Leeuwenhoek to the Royal Society, dated at Delft Observations of the circulation of blood in tadpoles: blood vessels; clotting; computation of quantity of flowing blood, capillaries Red-chalk figures attached to EL/L3/12 Read to the Royal Society on 29 November 1699 EL/L3/10 for covering letter EL/L3/12 for English translation Transcribed in JBO/10/152 Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, vol 12, p 335 English translation printed in 'Philosophical Transactions', vol 22, no 260, p 447 25 September 1699