A shudder in the loins engenders there The broken wall, the burning roof and tower
Since "Leda and the Swan" is a Petrarchan sonnet (see "Form and Meter"), line 10 makes a shift called "the turn."
Here the shift is obvious: the swan completes the sexual act. "A shudder in the lines" means that either the bird or Leda has had an orgasm.
The swan "engenders" new life inside Leda; that is, he makes her pregnant.
Yeats once again assumes that his readers know the end of the myth: that Leda becomes pregnant with the beautiful Helen of Troy, over whom the famous Trojan War will be fought.
Troy was the site of an important battle in Greek mythology. It marked the transition between the ancient and modern worlds.
The Trojan War was also the source of inspiration for Homer's Iliad, one of the most influential epic stories in Western civilization.
Put briefly, the war was fought between the Greeks and the Trojans.
Helen was the wife of the Greek king Menelaus, but stolen away to Troy (perhaps willingly, perhaps forcefully) by a young prince named Paris. Menelaus enlisted the help of another, more powerful Greek king, Agamemnon, to besiege or "sack" Troy.
After a hard-fought battle, the Greeks completely destroyed Troy.
However, when Agamemnon returned home, he was murdered by his wife Clytemnestra.
As if things couldn't get more complicated, Clytemnestra was the daughter of Leda and her Spartan husband.
The poem suggests that by literally impregnating Leda with Helen, Zeus impregnated her "metaphorically" with the future consequences of Helen's actions: i.e., the Trojan War.
The "broken wall" and "burning roof and tower" refer to the famous burning of Troy. So, although Leda didn't directly cause the Trojan War, she is now one of the indirect causes.
And Agamemnon dead. Being so caught up, So mastered by the brute blood of the air,
(Note: we're considering line 11 to include both "And Agamemnon dead." and "Being so caught up." Otherwise, the sonnet would have fifteen lines, and sonnets by definition have only fourteen. Yeats breaks up the line on the page in order to add a pause and to transition to a new topic.)
The break between 10 and 11 is very dramatic. The poem essentially says that Leda is responsible for the burning of a city and also the death of a great king.
Although Agamemnon was one of the victorious Greek kings who actually won the Trojan War, he didn't get to enjoy his victory.
He was murdered by his wife Clytemnestra and the lover she took up while her husband was at war. (Clytemnestra was one of Leda's later children, not born of Zeus.)
The rest of the poem is a long rhetorical question.
The first part of the question begins after the break in line 11. The speaker notes how Leda was "caught up." Remember that in line 3 he describes how the swan had "caught" her with his bill.
Leda was "mastered," or overpowered, by Zeus's "brute" or animal nature. Being a bird, his brute blood is "of the air."
This last phrase might also lead us to think that the whole encounter takes place while the swan is flying.
Did she put on his knowledge with his power Before the indifferent beak could let her drop?
The speaker finishes his rhetorical question.
He asks if Leda was conscious or aware enough during the encounter to "put on" Zeus's knowledge with his power.
The entire poem hinges on this vague expression "put on." We see two possibilities.
The first possible meaning is: did Leda realize that Zeus had the omniscient "knowledge" of a god as well the incredible power of a giant bird?
She already knows about the power part, but did she also know she was dealing with a god whose knowledge extended far beyond a mere mortal's?
In fact, Zeus's knowledge might even extend to knowledge of the future – he knows that their daughter Helen will cause the Trojan War.
Did Leda sense that Zeus knew the consequences of his act?
The second possible meaning is: did Leda actually acquire some of Zeus's knowledge along with his power?
Obviously, she gains power in the sense that the fate of world history is now linked to her own.
But did she also gain or "put on" Zeus's all-seeing knowledge, including his knowledge of the future? Did Leda become, in some sense, a goddess herself?
With both these possibilities, there's an alternative, an "Or." Or did it all happen too fast?
Maybe there wasn't enough time for her to learn of Zeus's knowledge before he set her back on earth, having accomplished his goal.
Maybe she was too confused by the initial blow. Maybe she didn't even know it was Zeus who had raped her.
The exact wording of the last line makes Zeus sound especially cold: his "indifferent" or uncaring beak released her and "let her drop" on the ground.
One of the most interesting aspects of Ancient Greek religion was how the gods seemed to have their own little world, and every once in a while they would swing by the mortals' world to throw a wrench in the plans.
Literary folks like Yeats have long been fascinated with this strange dynamic between mortals and immortals in classical religion.
There's a lot more to say about this ending and the whole encounter, so visit the other sections of our Shmoop analysis to find out more.