Wrote Letter L-200 of 1688-09-07 to the members of the Royal Society about development of the eggs of green frogs, blood circulation in frogs and the tail fin of roach and bream, and the shape of red blood cells

September 7, 1688
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Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren

Leeuwenhoek wrote this letter about green and brown frogs, development of the eggs of green frogs and their metamorphosis, blood circulation and heart rhythm of frog from larva to adult, the structure of a small fish, blood circulation in the tail fin of roach and bream, and the shape of erythrocytes of fish and birds.

He addressed the letter to the members of the Royal Society, but Edmond Halley was editor at the time, so it was not published in Philosophical Transactions. The drawings he sent are lost.

He published the letter in a separate pamphlet, Omloop des Bloeds, and in the Natuurs Verborgentheden Ontdekt: zijnde een Tweede Vervolg der Brieven. Those versions used the unfolding plate below (click to enlarge), which contains the ten figures referred to in the letter.

Other researchers, including William Molyneaux in 1684, had used microscopes to observe the flow of blood. Leeuwenhoek was the first to show the red blood cells making the turn from arteries to veins via the capillaries.

At the end of the letter, he wrote (Alle de Brieven translation):

Some time ago when I was reporting to a certain Professor of Medicine my discovery relating to the circulation of the blood, this Gentleman told me that, when people were discussing my observations, and referring to them in confirmation of certain Matters, the response frequently was: are we to believe it just because Leeuwenhoek says so; what certainty do we have about it. For which reason that Gentleman warned me, and said that I would do well to produce an attestation of a few prominent persons who might have been eye-witnesses to these my discoveries, in order that I might suffer less contradiction on such related matters.

It is quite true that, from special considerations, I have not mentioned hitherto (in my letters) anybody by name, amongst those who, together with me, have seen with their own eyes some of the most remarkable things with the aid of my microscopes; but I only said in general that I had demonstrated the same to some Gentlemen with sound knowledge and judgment, Lovers of Natural Science.

But since I now hear that more credence will be given to what I say if I proceed to specify those who have partly seen the aforementioned circulation or course of the blood, concerning that about which I have now written and communicated to Your Honours, I will make no difficulty in mentioning, out of many, those names as I believe will merit the greatest confidence, such as Mr. Cornelis Schravesande, M.D., Full Lecturer in Anatomy and Surgery, Councillor and One-Time Magistrate. Mr. Cornelis Vallensis, LL.D., also Councillor and One-Time Magistrate. Mr. Antoni Heinsius, LL.D., Councillor and Pensionary of this City, and formerly Envoy Extraordinary to His Royal Majesty of France, and recently Commissary of this State to the Court of His Royal Majesty of England.

These Gentlemen, to whom I am accustomed to communicate many of my discoveries, I have shown, among other things, the veritable circulation of the blood, as distinctly as if we were looking with our naked eye at the movement of the water (in a running river).

Note the "out of many".