The pre-publication of parts of Huckleberry Finn in the Century Magazine resulted mainly from the determination of the editor, Richard Watson Gilder, who wanted to publish the whole novel serially, talked MT into an excerpt for his December, 1884, issue, then convinced MT to let him print two further installments, in January and February, 1885. MT participated in these publications, though apparently Gilder selected which parts to use, and apparently too did most of the expurgating by which aspects of the novel were "adapted to our audience."
For the December excerpt, Gilder chose the Shepherdson-Grangerford feud chapters (17 & 18), using a piece from Chapter 19 to lead into it, and getting MT to write a brief introductory in which MT refers to the novel as the story of "two fugitives" -- Huck and Jim. Only a few expurgations were made in this installment.
In November, Gilder attended a performance by MT and Cable in New York, where he probably heard MT enact at least one of the comic dialogues between Jim and Huck that occur early in the novel, about "speculatin' in stock" and "King Sollermun." It was presumably at that time that Gilder arranged to publish other parts of Huck Finn in the first two issues of 1885. In the January Century he printed those two dialogues together. Among the several differences between the Century version and the novel, the most significant, and to me ominous, is the way Jim's concluding remark about being rich is changed, from "I owns myself...I wish I had de money" to "I owns myself...but live stock's too resky, Huck; I wisht I had de eight hund'd dollars en somebody else had de nigger." That particular change is one the MT himself was responsible for, as we can tell from transcript of MT's December performance of the dialogue in Canada, as published by a reviewer for the Toronto Globe. According to his report, for a live audience the conversation ended with Jim saying "Still, betwixt you an' me, I don't like it invested in dat way. My experience in live stock is resky; makes no difference if its in cows or niggers, its resky, an' I would feel much more easy if I had de eight hundred dollars an' somebody else had de nigger."
The February excerpt was the longest, and the most heavily revised. It is an abridged version of Huck's misadventures with the King and the Duke (from Chapters 19-28). On the three pages included below (of 22 altogether) can be noted some representative alterations, including Gilder's efforts at cleaning up "The Royal Nonesuch." The installment comes to an abrupt end, but not before it is made clear that the two rascals will be exposed.