This is only a short answer space but I can provide a general comment. These chapters mark Huck's first moments of maturity. Up until this point, he followed the authority of those around him, such as Pap, the Widow, Miss Watson, Judge Thatcher, and the King and Duke. The moment Huck decides to steal the money, he breaks free of this authority. For the first time, Huck acts on his convictions and morals to help other people, rather than simply acting on his personal desires.
Huck's interaction with Mary Jane also highlights an emerging aspect of his growth, namely an interest in women. In The Adventures of Tom Sawyer, Huck viewed girls as nothing more than an annoyance and did not believe they were to be taken seriously. Here, in contrast, Huck calls Mary Jane beautiful, and comments that when he saw her light a candle in the window, his "heart swelled up sudden, like to burst."