While I have been out of action, a technology-world friend named Michael Jones has generously added to the world's store of knowledge on the Frog Question. He has the floor:

German physiologist Friedrich Leopold Goltz [left, Wikipedia image] published his studies of decerebrated frogs in Beitrage zur Lehre von den Functionen der Nervencentren des Frosches. (Berlin: August Hirschwald, 1869.) There, 140 years ago, he begat the familiar story of the slowly-boiled frog.
The key element of this scientific discovery, lost across the years in
the story's retelling, is that the frogs must first have their brains
removed.
Goltz work inspired George Henry Lewes--actor, philosopher, friend of Dickens, bigamous partner of Marian Evans (George Eliot) and of note, literary critic--to extend the slowly-boiled brainless frog oeuvre
by slowly-boiling frogs with partial brains or with their spinal cords
severed at various locations. Lewes published his findings four years
and many frogs later as Sensation in the Spinal Cord in Nature, Dec. 4, 1873. He summarized the story this way:
"Goltz observed that a frog, when placed in water the temperature of which is slowly raised towards boiling, manifests uneasiness as soon as the temperature reaches 25° C., and becomes more and more agitated as the heat increases, vainly struggling to get out, and finally at 42° C., dies in a state of rigid tetanus. The evidence of feeling being thus manifested when the frog has its brain, what is the case with a brainless frog? It is absolutely the reverse. Quietly the animal sits through all successions of temperature, never once manifesting uneasiness or pain, never once attempting to escape the impending death."
Countless slow-boilings of partially dismembered frogs by Goltz, Lewes, and numerous others conclusively show the following truths: first, that even a brainless and spineless frog will recoil from hot water; and second, while healthy frogs will jump out of water when the temperature slowly gets too hot, brainless or spineless ones will not. The general sense of the slowly-boiled frog metaphor thus echoes scientific fact, even with its factual basis--elision of the frog's brain--itself elided through time and retelling.