Kilayim, chapter 3, mishnah 6. This mishnah deals with someone who has a field sown with vegetables and wants to plant several rows of gourds within it. Gourds, as has been mentioned several times already, tend to spread out with their leaves and become intermingled with other plants, and therefore the mishnah requires proper separation between the species being sown.
The case in the mishnah:
"Sadehu zarua betzalim umevakesh litah betocho shurah shel dela'im" - a field that is entirely sown with onions (and the same would apply to any other kind of vegetable, only that the mishnah chose onions as its example), and its owner wants to plant rows of gourds within it. Note that the rows under discussion in the mishnah are four amos wide, and all the calculations that follow are based on this.
The view of Rabbi Yishmael:
"Oker shtei shuros" - he uproots two rows of onions, thereby clearing an area of eight amos.
"Venotea shurah achas" - within those eight amos he plants only one row of gourds, four amos in the middle. The result is that two amos of separation remain on each side, between the new gourds and the existing onions.
"Umaniach kamas betzalim bimkom shtei shuros" - from there on he may leave two rows of onions, eight amos, standing and growing in their place.
"Ve'oker shtei shuros venotea shurah achas" - and when he wants to plant an additional row of gourds, he again uproots two rows (eight amos) and plants four amos in their middle, and so he continues in the same fashion.
Alongside the mishnah there is an illustration from the Gitim Mishnayos, which demonstrates the view of Rabbi Yishmael.
The view of Rabbi Akiva:
Rabbi Akiva holds that there is no need to be so stringent: "Oker shtei shuros venotea shtei shuros" - he uproots two rows of onions, eight amos, and fills that entire area with eight amos of gourds, with no measured separation between the gourds and the onions. Of course, the ordinary furrow mentioned in mishnah 3 of this chapter is required - the furrow found between any two kinds of vegetables, and apparently also between rows of the very same kind. Beyond that, no further separation is needed. And here too, afterwards he may leave two rows of onions in their place, and then uproot two more rows and sow in them eight amos of gourds and cucumbers, and so on.
The difference between Rabbi Yishmael and Rabbi Akiva is this: Rabbi Akiva holds that there must be a gap between the rows of gourds, so that it should not look like a field of gourds intermingled with a field of onions, and a clear distinction is required - but the gap between the onions and the gourds themselves need not be of any particular measure.
The view of the Sages:
The Sages hold that if there are not twelve amos between one row of gourds and the next, one may not maintain another species between them, meaning that one may not leave the onions in the middle. The measure of twelve amos is derived from Rabbi Yishmael's calculation: since he requires two amos between the gourds and the onions, and eight amos of onions stand in the middle, adding two amos on each side brings the distance between the rows of gourds to twelve amos.
According to the Sages, however, one may fill that entire twelve amah gap between the two rows of gourds with onions, and the two amos of separation that Rabbi Yishmael required are not necessary. It is enough that twelve amos separate the two rows of gourds, and the whole area between them may be filled with onions. The approach of the Sages thus turns out to be a kind of middle position between the view of Rabbi Yishmael and the view of Rabbi Akiva.
In summary: in this mishnah the Tannaim disagree about a field of onions in which one wishes to plant rows of gourds. Rabbi Yishmael requires uprooting two rows (eight amos) and planting only one row in their middle, so that two amos of separation remain on each side. Rabbi Akiva permits uprooting two rows and filling all eight amos with gourds, with no measured separation apart from the ordinary furrow, provided that there is a distinction between the rows of gourds. And the Sages rule with a middle approach: as long as twelve amos separate one row of gourds from the next, the entire gap between them may be filled with onions.