Uncertain of what the phrase "The Mighty Yawn that gave you birth..." means in Fitzgerald's This Side of Paradise

Solution 1:

I think Fitzgerald intends to establish a parallel between Mighty Yawn and Great Breath, being satirical about the creative activity described in Genesis where the breath of God gives birth to man.

My interpretation of the last paragraph: The lecture hour is up and the students file out as soon as they can (before the last word or two is spoken, "cheating" the lecturer of that last bit of time). As they exit noisily, the lecturer (and by extension, the lecture itself) is so absent from their minds it's as if they've forgotten the lecturer even existed.

Solution 2:

Fitzgerald is not always an easy read. That speech reminds me a bit of the way Cyrano de Bergerac would skewer his opponents with words (usually just before he did it with his sword).

I think he's saying that the Professor's birth was nothing more than a mighty yawn - something of little significance.