The usual style of the gemara is to have one mishna and bring the commentary and then another mishna and commentary. Sanhedrin starts out oddly bringing the entire 1st perek of mishna on daf beis amud alef, six mishnayos. And then bringing the commentary on all of them. (The Yerushalmi does not do so.) Why did Ravina and Rav Ashi choose to arrange the 1st perek of Sanhedrin this way?
Menachem Weiman, St Louis, MO
Actually, there are quite a number of Perakim for which an entire Perek of Mishnayos was "packed" into the beginning of the Perek in the Gemara, instead of being split throughout. Here is what I can recollect:
Berachos - 8, 9
Ta'anis - 2, 3, 4
Yevamos - 14
Sotah - 4, 5, 6
Sanhedrin - 1,5
Shevuos - 1, 2, 5, 7, 8
Tamid - 2, 4 (there is no Gemara at all for 3, 5, 6, 7)
A few of these chapters are not very long, or do not have much Gemara on them (Berachos 8, Yevamos 14, Sotah 6, Shevuos 5 and 8, Tamid 2,4), which is why their Mishnayos may not have been split up. But that does not suffice to explain many of the Perakim on this list. Nor does it explain why other short Perakim do have their Mishnayos split (e.g. Shabbos 13, 21).
Rashi in the beginning of Vayikra explains that the breaks between Parshiyos (i.e. splits) were meant to give Moshe Rabeinu - and us - an opportunity to stop and think over what was learned. That may be why Rebbi indeed split the Mishnayos into separate "bite-size" Mishnahs and chapters, instead of long uninterrupted tractates. And that may be why the Mishnayos normally are used for breaking up the Gemara into separate sections as well. So, in short, I have no clear answer as to why these chapters were not broken up.
Nor am I certain that it was Ravina and Rav Ashi's choice to split the Mishnayos this way. For all we know, the Gemara they redacted was simply a long run-on for each chapter. It was the printers who put the Mishnayos where they belonged - most of the time. In the scrolls used by the Rishonim, the Mishnayos may have been placed in the Gemara very differently. This is evident from the many places where Rashi's commentary on an earlier Mishnah only appears on the following Mishnah - because the printer did not make sure that Rashi's commentary goes where it belongs (for example, see Rashi end of Ta'anis ch. 3, Daf 25b DH Kodem etc.).
Kollel Iyun Hadaf