In the second stage of development, the embryo undergoing a replication called mitosis. In mitosis, every cell will comes from a previous cell and it has the exact same information that the cell it came out of.
If one of those cells of the embryo mutates, later it will divide making the subsequent cells also with the same mutation. This mutations can explain genetic disorders in which the child is affected but it doesn't run in the family.
Because the if it didn't slow down, the capillaries couldn't handle the high pressure of the blood.
Same shape, but not the same size.
