what is the signifigance of bucks transformation as he goes into the forest ?

Despite Buck’s great love for Thornton,  the "strain of the primitive" remains alive and active in Buck. He is a wild dog who sits by John Thornton's fire. With his final debt to Thornton is paid, he rips apart the Indians responsible for killing Thornton, Buck hears the cry of the wild wolf. Buck now understands what the call meant, running wild and free with his brothers,

"Deep in the forest a call was sounding, and as often as he heard this call, mysteriously thrilling and luring, he felt compelled to turn his back upon the fire..."