Letter, from Antoni van Leeuwenhoek to Christopher Wren, dated at Delft Signed autograph Theorising about the procreation of frogs; observations of the spermatozoids of the frog; trying to find spermatozoids in a fowl's egg; concerning the reproduction of rabbits and that hereditary characteristics pass down the male line; observations of a dissected frog; theory about digestion in humans and in fish; stating that blood does not ferment and then discusses blood circulation Original figures no longer exist Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, vol 4, p 53 Translation printed in 'Philosophical Transactions', vol 13, no 152, p 347 16 July 1683