1. Two rectangular fields may meet one another at a vertex, since each one is ראש תור for the other. “Jailed” means here that a rectangular field forms ראש תור at all four of its vertices. Rebbi Meïr permits to sow rectangular fields, each of the minimum size of bet rova‘, even if they are adjacent, i. e., they are joined at an edge, and even if they are jailed at all corners, which means that he permits to use the entire field. The Sages do not disagree that a קרחת must have a minimum size of bet rova‘. If the field is divided into equal rectangles, one may imagine that the different rectangles are alternately black and white in checkerboard fashion. Since the rabbis do not permit adjacent rectangles to be sown, the first condition is that only rectangles of the same color are chosen. The second condition is that no chosen rectangle can be surrounded by 4 chosen rectangles. The opinion of Maimonides about the position of the rabbis was explained in the Mishnah. His opinion is difficult to accept, since he does not permit any two fields to touch at all, so the mention of “jailing” is totally redundant. It is therefore better to look for a partition of the 50 by 50 field into 24 equal parts. The rabbis cannot speak about division of the field into squares since their number would have to be either 16 (with 7 chosen squares) or 25 (where 12 chosen squares are possible.) Also, the squares in the second case would be smaller than a bet rova‘. If the Sages would follow the division of R. Meïr and cover the field by a 4-by-6 checkerboard, they could accomodate 10 chosen rectangles (fields 1,3,5 in the first row, 2,4,6 in the second, 1,5 in the third, 2,4 in the fourth.) Hence, it seems that the rabbis insist that the area for קרחת should be strictly larger than a bet rova‘, and on a 5-by-4 board one may select at most nine (e. g., fields 1,3,5 in row 1; 2,4 in row 2; 1,5 in row 3; 2,4 in row 4.) The area of each plot is then 125 sq. cubits. (Explanation of R. Eliahu Fulda.)
2. This statement fails in two respects. First, the underlying checkerboard is 5-by-5, allowing only 100 sq. cubits per plot, and second, on such a checkerboard that has 13 white and 12 black fields, one may choose all white fields with the exception of the central one (#3 in row 3), for a total of 12 admissible fields. Hence, this sentence (choosing 3 plots in the first row, 2 in the second, 1 in the third, 2 in the fourth, 1 in the fifth) must be a later addition. Even if one chooses the central element in row three, one still could accomodate 10 plots (1,3,5 in the first row, 2,4 in the second, 3 in the third, 1,5,in the fourth, 2,4 in the fifth).
The argument is not part of R. Joḥanan’s statement since it is in Aramaic. R. I. J. Kanievski proposes to read תְּלַת תַּרְתֵּי וְחָדָא תַּרְתֵּיי וְחָדָא, eliminating two conjunctions and translating “three [times] two and one each.” The syntax is very unusual.