To review, the puppies' eyes and ears are sealed shut when they're born. Their eyes open first (usually at around 12-15 days old for our breed), and their ear canals usually open around 7 days later. That period, between the time that their eyes and their ears open, is known as the "transitional" period. This is because useful vision and hearing are thought to be necessary for the puppies to enter into the next developmental period, which will be the socialization period.
What's really important to know is that scientists use behavioral rather than time line markers to determine when different developmental periods begin and end. So it's the eyes opening and the startling that dictate the beginning and end of the transitional period, not "the third week of life" as those ubiquitous internet "Puppy Development" charts would lead one to believe.
This means that one puppy's transitional period might be when the puppy is from 10-21 days old, and another might be from when the puppy is 18-22 days old. I'm not kidding, it can vary that much, or more. The litter that Pippi's was born in was ridiculously slow to open their eyes - some of them had just started to open their eyes at 23 days old. I died a thousand deaths and was sure they had no eyeballs. But everyone turned out just fine :o).