Leeuwenhoek spent most of this letter responding to other researchers.
The first was a "certain author" that Alle de Brieven / Collected Letters noted was probably Charles Drelincourt. His book De foeminarvm ovis, tam intra testiculos & uterum quàm extra had been printed by Leiden bookseller Daniel van Gaesbeeck in 1684, the same year that Gaesbeeck printed six of Leeuwenhoek's letters, though not this one. In 1687, Gaesbeeck printed a second enlarged edition of De foeminarvm ovis.
I am well aware that there are some who utterly repudiate my ideas concerning generation. Thus a certain author recently published a small volume in which he enumerates seventy authors, all of whom had stated that all embryos, both of man and of beast, come from an egg, without the author of the book committing himself to any considered statement of opinion on the matter; so that he only shows that he has read a great deal about anatomy, and has remembered much of it.
Leeuwenhoek put such importance on primary evidence -- what he could see -- that he gave little credibility to those who relied on the writings of others. This case was different, however, because he knew that Drelincourt knew better. Not only had Drelincourt been to Leeuwenhoek's house and seen evidence to the contrary of what he wrote. But he lied about it, though Leeuwenhoek charitably ascribed it to a faulty memory.
But when he came to speak of me personnally, he let it seem to the world that his memory was not great; for he said: ‘I do not know that man’; yet there were some among his audience who knew very well that he had visited me in my house in this city to acquaint himself with my speculations and to view what there was to be seen.
I also know that the few authors I have read on the subject, and many other learned men, several of whom are experienced in anatomy, with whom I have spoken, have always declared that no male seed, whether from man or beast, enters the womb. They all say that it has never been found there, and that only a vapor from the male seed penetrates into the womb and that it is this which effects fecundation.
After explaining that opposing view, he gave his response.
As for me, I have always been convinced of the contrary, and have always felt certain that the male seed itself is discharged into the womb, and that this is the reason why the male genitals of many animals must be long and thin, so that the seed may be carried right up to the womb and discharged into it.
He referred to his discussion with another leading authority, Nehemiah Grew, who himself referred to William Harvey.
I well remember that Mr. Nehemias Grew, Secretary of the Royal Society, when writing to me a few years ago, used the following words:
"For our Harvey, who absolutely denies (Lib. de Generat. Animal.) ever having found male sperm in the matrix, cut up immediately after copulation; and your Dr. De Graff, in the book De mulierum organis, has boldly established that the male sperm is nothing but the vehicle of a certain Sal Volatile or like spirit, impressing on the conception, i.e. the ovum of the woman, the perception of life (contactum vitalem)."
Leeuwenhoek then defended his observations. If he saw it, it must be true.
But let our author add his witness to his seventy authors; nay, let him (if he can and will) produce seventy times seventy more, all of whom uphold the ovary or egg-nest theory, and contend that the male seed is not discharged into the womb, I maintain that every one of them has erred, and that all still err who say that both man and beast come forth from eggs, and that male seed does not enter the womb. Nay, I say that this is one of the silliest propositions current among physicians.
He did not just speak poorly of them, he went on to recount yet another observation affirming what he had been seeing and reporting for years.
Johannes van Dueren
The second researcher whom Leeuwenhoek responded to in this letter was Johannes van Dueren, a medical doctor and ex-magistrate from Gorichem, near Rotterdam. He contributed short pieces to Stephen Blankaart's Collectanea medico-physica, oft Hollands jaar-register der genees- en natuur-kundige aanmerkingen, published periodically from 1680 to 1688. The "Cent. 5" that Leeuwenhoek refers to is Centuria V for the year 1681. Van Dueren's short paragraph on Leeuwenhoek is titled Wondere dingen in yders aard (Wondrous things in everyone's nature.)
I recently came across a small volume entitled Collectanea Medico Phisical, in which the following sentence occurs on page 8, Cent. 5.
"Most amazing of all, however, is that our learned Cornelis Bontekoe cites the ever curious Leeuwenhoek as asserting that human sperma is replete with small babies, and so likewise in other animals, each according to its kind."
He was referring to the famous "tea doctor" who had lived in Leiden until 1681. Leeuwenhoek wrote about Bontekoe with some irritation.
It is true that Mr. Bontekoe has often visited me in company with others; but never have I told him or anyone else in the world that the human semen is full of small babies.
What I did say was that it is full of living little animals or tiny worms which have long tails, and I have occasionally shown a drawing of their structure. For, just as we have no reason to say that some worms, while they are still swimming in water, are flying creatures, though creatures with wings will eventually emerge from them; or that the pip or core of an apple is a tree, though a tree will grow from it; it would be equally wrong to assert that the little worms in the human sperm are small babies, even though a child is formed from such a small worm.
I must say that I feel most aggrieved at the way my arguments are distorted or maliciously misrepresented, and worse still that, by being thus committed to print, they gain common currency.