How was semi conservative replication proven?

2 scientists used cultures of e. coli bacteria, and were given nutrients with either heavy nitrogen (15 N) or a light nitrogen (14 N). After the bacterias used the nitrogens as nitrogenous bases, they became heavy and light; now they can be compared. The bacteria were allowed to reproduce with the heavier bacteria twice. What was discovered was that the parental bacterai was at the bottom of the tube, then gen 1 was in the middle, and by gen 2 there were 2 layers: one lighter than the other. If it was conservative replication, 1 half would have been a heavy band and another a light band since the original would've still existed.