Wrote Letter 56 of 1687-07-11 (AB 101) to Members of the Royal Society

July 11, 1687
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Leeuwenhoek's summary

From all editions of Vervolg der Brieven and translated into Latin for all of the editions of Continuatio Epistolarum.


Dat de Zijd-worm-Eyeren blaauwagtig werden, is alleen door een dunne menbrane die van binnen tegen de schors van het Ey aan leyt. Waar het dierken uyt het mannelijk Zaad zich aan het Ey plaatst. Een seer kleine Zydworm in het Ey. Schielijke grootwerdinge van de Zyd-wormen in de Eyeren. Vaten of anderen in de ongeboorne Zydwormen. Dop van het Ey. De menbrane die in't Ey is, gaat tot het lighaam van de Zijd-worm over. Van den beginne der Schepping, is de levende Ziel van het Dierke van 't mannelijk zaad van de Zijdworm al in geschapen. Dat die eenige maanden in de Eyeren beslooten moeten leggen. Soo wy een jaar hebben dat 'er veel Rupfen zijn, dat het geen gevolg is, dat het naaste jaar daar weder veel Rupfen sullen komen. De Zyd-wormen komen des morgens en niet des naar-middags uyt haar Eyeren. De Zyd-wormen byten haar Eyeren ontstukken door behulp van een vochtige stoffe die zy uyt haar lijf weeten te brengen. Onbedenkelyke volmaaktheid in defelve. Rupfen die in de Na-Somer in haar Eyeren volmaakt waren, en echter in de maant Mey eerst uyt haar Eyeren quamen kruipen, Waarom de Menfchen scheel sien, en niet de Dieren. Acht distincte Planten in een Gorst Graan.


That the silk worm's eggs were bluish, is only due to a thin membrane that lies inside against the egg. When the little animals from the male seed place themselves on the egg. A very small silkworm in the egg. Rapid growth of the silkworm in the egg. Vessels or otherwise in the unborn silkworm. Shell of the egg. The membrane in the egg, going over the body of the silkworm. From the beginning of creation, the little animal of the male seed of the silkworm is already endowed with a living soul. That for several months must lie closed in the egg. So we in one year have many caterpillars there, that it is no result that the next year there again many caterpillars will come. The silkworms come out of their eggs in the morning and not in the afternoon. The silkworms bite their eggs [inflamed?] with the help of a moist substance that they bring out of their bodies. Unimaginable perfection of the same. Caterpillars that in the fall are perfected in their eggs, and yet in the month of May creep from their eggs, Why people squint, and not animals. Eight distinct plants in a barley grain.

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.

The original manuscript, written and signed by Leeuwenhoek, is preserved at the Royal Society (MS. 1922. L 2. 13).

There is no record in Birch's History that this letter was received or read at a weekly meeting.

Response to other researchers

Reproduction and growth at the beginning of life was an ongoing concern for both Leeuwenhoek and the Royal Society. Leeuwenhoek opened this letter by reminding the members of the Royal Society that they had asked him to examine silkworm eggs.

In Your Honours' missive of the 17th June 1685 it was, among other things, recommended to me to examine the fertile and sterile eggs of the silkworm. For that purpose I acquired during the past early summer eggs of the silkworm. But since by that time the silkworms in the eggs had grown so much that they came crawling out of their eggs, I postponed my observations, until I had received such eggs as had been laid only a short time before. By chance, I came to talk with the person whose children had been breeding silkworms.

Specimens and methods: silkworms in his pocket

Leeuwenhoek reported that he and his wife carried silkworm eggs on their bodies to keep them warm.

On the 10th September I placed the silkworm eggs that were laid six weeks earlier, in a small, flat screwed-up box, and I carried that in my pocket during the day, and took it to bed with me at night, so as to keep them warm all the time. And in another similar box I put eggs that were three weeks old. These eggs, my wife (who clothes herself very warmly) carried in her bosom night and day, for this purpose, that I might watch the silkworms grow from time to time in the autumn (if that were possible).

He did not note whether these screw-top boxes were wood or metal.

I then proceeded to open the silkworm eggs that my wife had been carrying about with her, and therein I found the silkworms, seen through the microscope, to be quite as big as an ordinary finger; and I had the intention to have them drawn. But when the animal had dried, the same had dried up so irregularly ... that I could not have perceived either head or tail, or any further division of the body, if I had not seen them quite distinctly in the first instance.

These observations carried over to the following year.

On the 14th May, I again opened some eggs, and I then saw that the silkworm increased in size; at this time I was carrying several eggs in a screw-top box in my pocket. After a lapse of four days I again opened these eggs, and then saw that most of the moist substance in the egg had united with the rest and I was then able to see the ringed parts of the silkworm's body bigger than before.


The original drawings are lost. The Dutch and Latin editions that Leeuwenhoek published used the same plate with both figures. The one below (click to enlarge) came from the 1730 fourth edition of Continuatio Epistolarum, as did the images on the sidebar. In the text, Leeuwenhoek noted that someone else drew the figures.

Plate from
Continuatio Epistolarum

Figures 1 and 2

Limitations of his figures

As he had in the previous letter, in this letter he mentioned the visual acuity of his draughtsman. After explaining Fig. 1, he added:

When I dissected the body of a silkworm, which was still mainly transparent, I discovered a still much greater number of small vessels or stripes.

He had trouble seeing them himself.

Here, too, I intended to have such a silkworm drawn. But no sooner had the moisture evaporated from the same, than the vessels, which one had seen so distinctly before, now were irregular, and many could not be discerned.

What ended up in Fig. 1:

The blackish vessels (so far as the draughtsman could perceive them) that lie on one side of the head of an unborn silkworm.

Leeuwenhoek recognized the problem:

My intention was, indeed, to indicate also the vessels that lie on the other side of the head. But when the draughtsman was going to draw them, I saw that he took to be vessels the cracks and fissures that had appeared in the substance due to the moisture having dried up, and therefore I stopped my intention.

In Letter 55, the problem could have been caused by the difficulty in focusing Leeuwenhoek's devices. Here, however, the problem seems to be the draughtsman. This passage makes it clear that he drew what he saw, not necessarily what Leeuwenhoek saw.

Other publications

While the Royal Society did not publish this letter in Philosophical Transactions, it was extracted twice in foreign journals, as were all the letters in Vervolg der Brieven, Letters 53 through 60. In the year after it was written, Jean Le Clerk published a long excerpt, without any figures, in Bibliothèque universelle et historique, vol. 9, pp. 301-311.

In 1689, Otto Mencke published a very short extract, again without figures, in Acta eruditorum, vol. 8, p. 172.