13. Among the thirteen natural numbers from $1 \sim 13$, select twelve natural numbers to fill in the 12 empty cells in the figure, so that the sum of the four numbers in each row is equal, and the sum of the three numbers in each column is equal. What is the number that is not filled in the cells? And please fill in the other numbers in the cells.

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【Analysis】Considering the row, we have 3 and $=1+2+\cdots+13-a=91-a$, which means $a$ leaves a remainder of 1 when divided by 3. Considering the column, we have 4 and $=1+2+\cdots+13-a=91-a$, which means $a$ leaves a remainder of 3 when divided by 4. Thus, $a=7$, and the sum of the row is 28, and the sum of the column is 21. When filling in, consider that $10, 11, 12, 13$ cannot be in the same column.
By the pigeonhole principle, at least 2 of these 4 numbers must be in the same row.