For undis as a corrupt hexameter ending, compare Met XV 276 'redditur Argolicis ingens Erasinus in aruis [codd: in undis Sen NQ III 26 4]', Met VIII 162 'liquidus Phrygiis Maeandros in aruis [uar liquidis Phrygius ... in undis]', and Met XIV 155 'sedibus Euboicam Stygiis emergit in urbem [uar sedibus euboicis stigiis emersus ab undis]'.
The line seems to have passed without comment until Merkel's second edition: 'in undis minus bene positum uidetur; temptabam hiulcas, quod expressisset Statius Theb. VIIII 450 hiulcis flumina uenis Suggerit ['he (the river Asopos) opens his springs wide and adds his streams']'. There seems no obvious reason, however, for Ovid to define the springs as 'gaping'.
Madvig conjectured INVNDANS, the corruption of which would be easy; but uenas seems more in need of a modifier than limus—Professor R. J. Tarrant suggests APERTAS or AQVARVM, Professor A. Dalzell IN ARVIS.
Professor Tarrant also suggests to me that in undis could well have originated as a gloss on uenas.
18. LAESAQVE. There seems no reason to replace this with Merkel's LAPSAQVE ('flowing back'?), which even seems to contradict the sense of resistit.
The same sense of laesus at Am III vii 32 'deficiunt laesi carmine ['spell'] fontis aquae'.
20. VENA PAVPERIORE. The same image of Ovid's poetic talent at Tr III xiv 33-34 'ingenium fregere meum mala, cuius et ante / fons infecundus paruaque uena fuit' and EP II v 21-22 'ingenioque meo, uena quod paupere manat, / plaudis, et e riuo flumina magna facis'.
23. DA VENIAM FASSO. As a poet himself, Severus would be particularly shocked at Ovid's admission he has virtually ceased to write poetry. Similar phrasing at III ix 45-46 'confesso ignoscite, docti: / uilior est operis fama salute mea'.
23. FRENA REMISI. 'I have let go of the reins' = 'I have stopped writing poetry'; for the sense, compare Aen VII 599-600 (of Latinus) 'nec plura locutus / saepsit se tectis rerumque reliquit habenas'.
The metaphor of the poet as driver is found as early as Bacchylides (V 176-78) and Pindar (Ol VI 22 ff). A full list of Greek and Latin passages is included in Henderson's note on RA 397-98; the image is particularly frequent in Roman didactic poetry, being found even at Columella X 215-16. See as well Kenney Nequitiae Poeta 206. In Ovid the image is found at AA I 39-40 & 264, II 426, III 467-68 & 809-10, RA 397-98, Fast I 25-26, II 360, IV 10, and VI 586. The only instances I have found that are not from Ovid's didactic verse are the present passage and xii 23-24 'tu bonus hortator, tu duxque comesque fuisti, / cum regerem tenera frena nouella manu'.