Who or what does Buck find in the forest in the middle of the night in The Call of the Wild? What happens upon making this discovery?

In Jack London's The Call of the Wild, Buck the dog is sold and becomes a sled dog, eventually coming into the hands of Jack Thornton. They travel together into the Alaskan wilderness, where Buck discovers the thrill of being free to explore the forest.
Buck enjoys this freedom in chapter seven, as he hears a wolf's howl in the distance in the middle of the night and chases after it. After Buck's cautiously friendly approach, the wolf bolts, and Buck spends a good while chasing the wolf, cornering it to try and gain its friendship. Buck discovers a kinship with this wolf, as seen here:

But in the end Buck's pertinacity was rewarded; for the wolf, finding that no harm was intended, finally sniffed noses with him. Then they became friendly, and played about in the nervous, half-coy way in which fierce beasts belie their fierceness. (Chapter 7)

With the friendship established, Buck follows the wolf through stretches of woods until well into the daylight. This adventure seems to be an answer to Buck's earlier musings about what it is to be wild and free.

Buck was wildly glad. He knew he was at last answering the call, running by the side of his wood brother toward the place from where the call surely came. (Chapter 7)

This moment of discovery for Buck bears significance in regard to the title of the book—The Call of the Wild. Buck seems to find the source of the call, and the joy in answering it beside his "wood brother," the wolf. It is a celebration of untamed freedom, and this moment leads to a change in Buck: he begins to embrace the "call," transforming into a wild beast rather than a tamed sled dog.