Letter, from Antoni van Leeuwenhoek to the Royal Society, dated at Delft Signed autograph Observations of green and brown frogs; development of the eggs of green frog and metamorphosis; blood circulation and heart rhythm of frog from larva to adult; structure of a small fish; blood circulation in the tail fin of roach and bream; the shape of erythrocytes of fish and birds Original figures no longer exist; engraved figures for publication are attached (found after EL/L2/23) Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, vol 8, p 3 7 September 1688