I wanted to point out a detail in this transition:
“Espied on a pine’s bark,
As we were walking home the day she died,
An empty emerald case, squat and frog-eyed,
Hugging the trunk; and its companion piece,
A gum-logged ant.
That Englishman in Nice,
A proud and happy linguist: je nourris
Les pauvres cigales—meaning that he
Fed the poor sea gulls!” (Lines 236-243)
The first scene takes place in "the beginning of 1950" the day Maud Shade dies.
The second scene is not so obvious on first read, but it's a reference to Hazel's conception in Nice in 1933. Shade gives us this information a little later in the poem when he repeats the image of the man feeding the gulls, this time connected with Hazel's birth:
“And then there was a kind of travelog:
A host narrator took us through the fog
Of a March night, where headlights from afar
Approached and grew like a dilating star,
To the green, indigo and tawny sea
Which we had visited in thirty-three,
Nine months before her birth. Now it was all
Pepper-and-salt, and hardly could recall
That first long ramble, the relentless light,
The flock of sails (one blue among the white
Clashed queerly with the sea, and two were red),
The man in the old blazer, crumbing bread,
The crowding gulls insufferably loud,
And one dark pigeon waddling in the crowd.” (Lines 429-442)
Once we've backpropagated this information, we can see that we've transitioned, in the middle of line 240, from Maud's death in 1950 to Hazel's conception in 1933 by way of a coincidence centred around cicadas: on both occasions, shade is reminded of La cigale et la fourmi by La Fontaine (spelled Lafontaine in PF), first with the chance pairing of a cicada's case and an embalmed ant; next, the mention of "pauvres cigales" reminds Shade of La Fontaine because the Englishman mistranslates sea gulls as cigales, which word really means cicadas, though translators of La Fontaine likewise mistranslate cigale as grasshopper in The Ant and the Grasshopper (see note to line 238).
I thought I should point out that 17 years have passed between Hazel's conception and Maud's death, between cicada and cicada, which brings to mind a 17-year periodical cicada. Whether that was intentional, I can't be sure (though I worry an entomologist might confirm that the exuviae of periodical cicadas are NOT emerald). In any case...