The Greek text of the five books of Irenaeus Adversus Haereses is lost, aside from quotations. A Latin version exists, created in antiquity, and also an Armenian version of books 4 and 5. The French Sources Chrétiennes text contains some interesting statements about the tables of contents prefixed to each book, and chapter divisions.
In Sources Chrétiennes 100 (introducing book 4), p.42-3 we have the following statement by B. Hemmerdinger:
The Armenian version is not divided into chapters. But book 4 is preceded by a summary, while book 5 is not.
In Latin, book 4 is preceded by a summary and divided into chapters , while book 5 has neither. This implies that in Latin the chapters were created from the summaries, and that the Greek archetype of the Latin and the Armenian had a summary before book 4, but not before book 5.
This summary does not vary, either in the manuscripts or in the early editors from Erasmus to Grabe. It is an innovation in the Latin version to copy these argumenta and insert them in the text as titles of chapters. So there is no need for us to encumber the text of Irenaeus with these titles, which don’t belong there.
 In 80 chapters in the manuscripts…
On pp.186-191 there is a lengthy “Observation on the argumenta“.
…all the Latin manuscripts precede book 4 with a list of “argumenta”, which are then repeated in the body of the same book, a few variants aside, as titles for divisions of the text, divisions of very varying lengths. The Armenian manuscript offers a list which is substantially identical preceding book 4, but, differing here from the Latin manuscripts, the “argumenta” are not reprised in the interior of the book. This is the first piece of data which is imposed on whoever studies the “argumenta” in either version of the text.
This invites us to make a distinction between the list of “argumenta” on the one hand and the insertion of them in the body of the text on the other. The list of “argumenta” preceding book 4 is therefore anterior to the Latin and Armenian translators, since both of them translated it. Thus it belongs to the Greek tradition, even if, as seems certain, it does not go back to Irenaeus himself. As for the introduction of “argumenta” in the body of the text, it is more recent. Although it precedes the division of our Latin manuscripts into two families, it seems to be much later than the translation itself. This is clear from the fact that it does not respect the periods and phrases of the text, as Pitra already noted in 1884. The translator would hardly have brutally cut these in half, as in the case of V, XL, XLII, XLIV, XLVII, LVI, and LXXIV. … The insertion must be foreign to the Greek archetype common to the two versions, for otherwise it is inexplicable that there is no trace of it in the Armenian version.
What do these “argumenta” represent? As F. Sagnard has justly noted (SC34, p.78), this is not a division into real chapters, but more an overview of subjects treated, of a series of landmarks punctuating a course of progress quite often alien to the development of the work. Indeed rather than defining in a neat manner a step in the thought of Irenaeus, and seeking to summarise it personally, the author of the “argumenta” preferred, in a general fashion, to pile up this formula and that which struck him in the course of going through the text, and repeated them in compiling the list. In consequence there are a good number of “argumenta” which echo phrases in the Adversus Haereses, but where we look in vain for any development of the idea (e.g. “argumenta” VIII, IX, X). Hence also a certain daftness in the list. The author dwells unduly sometimes on pages that do not demand it, and sometimes ignores material which deserved a special note. All this shows that he had no intention of compiling a list of chapters in the modern manner. …
It does not follow that the list is thereby deprived of interest, and we believe that the too severe verdict of F. Sagnard should be revised. …
He then goes on to point out that because it derives directly from the Greek text, it can be used as a guide to correct transmission errors. Then there is discussion of the differences between the Latin and Armenian versions of the list, due to mistakes by the translator, or Latin or Armenian copyist errors, and substantial lacunas.
The comparison of the Latin and Armenian lists furnishes us a third piece of data. The numbering of the items in one is quite independent of that in the other. For the Armenian one can say that the numbering, made in the margin, seems to be the work of a later hand. This tends to show that the numbering formed no part of the early Armenian text, and was just added ad-hoc later on. Just by considering the numbering of book 4, we are driven to conclude that numbering did not form part of the Greek text. However this conclusion is weakened if we step outside book 4, which is our present study. For book 2, in fact, in Vaticanus 187 (Q), the “argumenta” are listed with a numbering in Greek numbers. J. B. Pitra drew attention to this and reproduced it in his Analecta Sacra vol. 2 (1884) p.215. Also the lemmas of three Syriac fragments, one of book 2 (Harvey II, p. 435, n. 1) and the first two of our book 4 (Harvey II, p.443, n.1; p.444, n.1) also attest to the existence of one and even many numberings, the origins and value of which we cannot discuss here. These observations may support the idea of numbering in the Greek text. But it must not be forgotten that book 5, in whatever version, manifests a kind of incompleteness in that it has no “argumenta”. In the era in which translators and compilers were using the book of Irenaeus, the Greek manuscripts must have presented, in the fragile portion which the initial list is, important lacunas and divergences, which the differences of the Latin and Armenian only reflect.
The author adds two studies on the subject of the Armenian argumenta: A. Merk, Der armenische Irenaeus Adversus Haereses — IV. Das argumentum des 4. Buches, ZKTh 50 (1926), p.481-494; and J. A. Robinson, The Armenian capitula of Irenaeus Adv. haereses IV, JTS 32 (1931), p.71-4.
In Sources Chrétiennes 152 (introducing book 5), p.30-31 we find the following interesting statement:
A peculiarity of book 5 in the manuscript tradition is the absence of argumenta, and consequently of chapters. The absence is a feature of the Armenian version as well as the Latin version, which suggests that the Greek copies which served as a basis for each were likewise devoid of argumenta. Why? Was it just laziness by the scribe originally charged with compiling them? Or an accident to a Greek archetype? It is not our intention nor within our power to pursue this question. But it is worth knowing that editors have reacted very differently to this absence.
It is worth mentioning that book 5 does have a preface.
In Sources Chrétiennes 34 (introducing book 3) the discussion is on p.77-8.
It was stated earlier that the argumenta found at the start of the books are the same in all the manuscripts. This is particularly so for book 3, where the list comprises 46 chapters. Loofs has already demonstrated in tabular form the agreement of the lists found in the manuscripts. However he made a mistake in assigning V a different numbering system. The error is simple: the numbers initially appear before the title to which they relate, and so at the end of the preceding title, but later on, because of long titles covering more than one line, they appear at the end of their own title.
The scribes have made many errors, which are easy to spot. …
The remainder of the comments are of a similar kind to those in SC 100, although he dismisses the titles as of no value.
I also had a look at SC406, which publishes Irenaeus Proof of the apostolic preaching, extant only in Armenian and found in the same manuscript (Yerevan 3710) as books 4 and 5 of Adversus Haereses, which it follows. The table of contents is mentioned for book 4 of AH, the lack of it for book 5, and no mention of one is made for the Proof.
I will perhaps look at books 1 and 2 tomorrow.