When I am standing in a subway car or walking down a crowded street, I often like to imagine everyone I see as newborn babies--old ladies with walkers, serious men in dark suits reading the Wall Street Journal, teenagers, nurses in scrubs returning from the Hospital--I like imagining what each person might have been like as a baby.

Tomorrow I take my third exam of medical school, in embryology. Thanks to embryology, my imagination can now make an additional leap. I can sit in a crowded subway car, and imagine each of us as a single cell, that became two, then four, then eight. Three thin layers. A tube. Each of us--a single cell! And I see people playing sudoku, listening to music, talking to their friends in the next seat, working on their laptops. A single cell.