Solution: The first day 2, the second day 3, the third day 4, the fourth day 5+ 1 (the one that split on the first day matured and began to split), the fifth day 6+2+ 1 (similarly, the one that split on the second day also began to split), and the sixth day 7+3+2+65438+. There's one on the eighteenth.

19+( 15+ 1)* 15/2= 139