This was a very exciting find - my first sprout with well-defined secondary leaves. When these open up over the next couple weeks, this little sprout will be ready for half-strength fertilizer to help accelerate its growth. By the end of this year, there should be around six small branches with secondary leaves.
In the zoomed-out photo, you can see how the stem leans to find sunlight. This is known as phototaxis. Every morning, I rotate the seedlings so they point away from the nearest window, and when I get home from work, they're pointed back at the window.