Summaries
None of Leeuwenhoek's self-published volumes has what we now know as a table of contents. Many of the volumes' title pages had summaries; published seprately, the title pages functioned as advertisements.
Most of the letters in the self-published volumes had summaries at the beginning, a list of topics in paragraph form with no references to page numbers. In several volumes, these summaries were collected in the front matter.
None of these summaries was part of the letters' manuscripts, so none are translated in Alle de Brieven / Collected Letters.
The numbering of volumes is Dobell's (1932).
Dutch editions
1. Onsigtbare geschapene waarheden
None of the letters in any edition had a summary. The first, Letter 32, had an italicized paragraph at beginning that looks like his later summaries. However, this one had the customary opening paragraph full of honorifics. This was the paragraph that the editors of Philosophical Transactions always omitted when they published their "extracts" of Leeuwenhoek's letters.
2. Eyerstok
It had a summary on the title page and the same summary with a couple of added phrases on the first page of the letter.
3. Schobbens in de Mond
It had a summary on the title page and the same summary with a couple of added phrases on the first page of the letter.
It also had a two-page dedication from the printer Daniel van Gaesbeeck to the reader (den Drukker aan den Leeser) dated July 24, 1684. On its second page (left sidebar; click to enlarge), it had summaries of the first three letters that Gaesbeeck printed but not the letter in Schobbens.
4. Humor Cristallinus
It had a summary on the title page and the same summary, with one short added phrase, on the first page of the letter.
5. Onsigtbare verborgentheden
It had summaries at the beginning of each letter.
6. Sout-figuren
Title page noted the most important topics in each letter. Then each letter had its own title or subtitle, long enough to be a table of contents or summary.
7. Zaden van Boomen
It had a summary on the title page and a longer summary on the first page of the letter.
8. Levende Dierkens
It had a summary on the first page of each letter. Each letter's title page (left sidebar; click to enlarge) also had "Ontledingen en Ontdekkingen" above the summary.
9. Cinnaber naturalis
It had a summary on the first page of each letter.
10. Vervolg der Brieven
It had a five-page Register der Saaken in the voorgaende Brieven vervat (Index of Topics contained in the foregoing Letters) (left sidebar; click to enlarge), a list of all Leeuwenhoek's previously published letters from 28 to 52, with their dates, publishers, titles, and summary of contents but not the dates of publication.
Right after the title, it has this NB:
De voorgaande 27 Brieven by den Auteur geschreven, en heeft hy tot noch toe niet konnen resolveren die met den druk gemeen te maken: dus hier de 28e Brief de eerste die gedrukt is.
The preceding 27 Letters written by the Author, he has until now not been able to resolve to make a public printing: thus the 28th Letter is the first that is printed here.
Each letter except the first (Letter 53) had its number followed by its addressee, and the word Handelende (dealing with) or Waar in gehandelt werd (Wherein is dealt with).
12. Tweede Vervolg
It had a one-page list (left sidebar; click to enlarge) of the main topics in the seven letters titled Eenige van de voornaamste saaken waar van dese Brieven handelen (Some of the main topics that these Letters deal with), summaries of the summaries at the beginning of every letter. There were no page numbers next to the items.
13. Derde Vervolg
Each letter had its number followed by its addressee and Waar in gehandelt werd (wherein is dealt with) or a slight variation of that phrase preceding the letter's summary. This styling (left sidebar; click to enlarge) was followed for the rest of the self-published volumes.
14. Vierde Vervolg
The title page had a hundred-word summary Handelende onder andere (dealing with among other things). Each letter except for Letter 81 (the reply to George Garden) had its number followed by its addressee and Waar in gehandelt werd (wherein is dealt with) or a slight variation of that phrase preceding the letter's summary.
15. Vijfde Vervolg
Each letter had its number followed by its addressee and Waar in gehandelt werd (wherein is dealt with) or a slight variation of that phrase preceding the letter's summary.
16. Sesde Vervolg
Each letter had its number followed by its addressee and Waar in gehandelt werd (wherein is dealt with) or a slight variation of that phrase preceding the letter's summary.
18. Sevende Vervolg
Each letter had its number followed by its addressee and Waar in gehandelt werd (wherein is dealt with) preceding the letter's summary.
19. Send-Brieven
The title page had a hundred-word summary. Each letter had its number preceding the letter's summary. The date and addressee followed, usually on the next page because the summaries were long.
Latin editions
21. Anatomia et Contemplatio
Letter 43, the first letter, did not have a summary; the other two did.
22, 23. Anatomia Seu Interiora Rerum / Arcana Naturae Microscopiorum
All of the 19 letters except Letter 43 had a summary.
24. Continuatio Epistolarum
All of the eight letters had a summary. In addition, these summaries as well as the letter summaries for Anatomia Seu Interiora Rerum / Arcana Naturae Microscopiorum were collected at in a three-part Index Rerum the beginning of the volume. In some bundles, Continuatio Epistolarum with the summaries of all the letters but itself containing only Letters 53-60 came between the two parts of Anatomia Seu Interiora Rerum / Arcana Naturae Microscopiorum or preceded the second part only, without the first part yet retained the full three-part Index Rerum.
25. Arcana Naturae Detecta
1695 edition
The first six letters in this volume were Letters 32, 33, 37, 39, 40, 41, the six published by Gaesbeeck in 1684 that Leeuwenhoek did not publish in Anatomia Seu Interiora Rerum / Arcana Naturae Microscopiorum or the Continuatio. Of them, only Letter 37 to Christopher Wren and Letters 40 and 41 to Francis Aston had short summaries. They were also unnumbered.
The rest, Letters 61-92, were numbered and had longer summaries.
1722, Edition Novissima, auctior & Correctior
This "latest edition, englarged and corrected" had a 14-page "Index Rerum / Epistonis Contentarum" with summaries of all 38 letters in this volume. These summaries were repeated at the beginning of each letter, including summaries for the three letters in the 1695 edition without them. As in the 1695 edition, the first six letters were not numbered and the lettesr from 61 to 92 were numbered.
26. Continuatio Arcanorum Naturae
The fifteen letters were numbered and had often long summaries. In the 1722 edition, but not the 1697 edition, these summaries were collected in a Index Argumentorum / Quae in Epistolis pertractantur (Index of Arguments that are treated in the Letters) on 8 unnumbered pages.
27. Epistolae ad Societatem
The 39 letters were numbered and had long summaries. These summaries were collected in a 15-page "Index Rerum / Epistolis contentarum" (Index of Topics contained in the Letters).
28. Epistolae Physiologicae
The 46 letters were numbered and had long summaries. These summaries were collected in a 15-page "Summarium Totius Operis / Sive Index materiarum, quae in his / Epistolis pertractantur" (Summary for the whole Work / Index of materials, which are treated in his Letters)