And to save our good doctor from having to repeat himself;
Shilolo said:
“There is a book by this title, FYI. The basic answer is that we all start the same as embryos. For the first 6 weeks of embryogenesis, we are all the same. Then, at around the 6 week mark (in intrauterine development), male sex hormones (mainly testosterone) are made due to the presence of the Y chromosome, that leads to masculine development.
You are still left with nipples and even a small amount of breast tissue from very early embryogenesis. Some men develop breast enlargement (called gynecomastia), and even breast cancer (about 1% of all breast cancer patients are men).”