High-resolution white light and H-alpha observations are used to study the development of 15 large active solar regions. The behavior of the magnetic fragments which coalesce to form mature spots and the nature of the subphotospheric flux rope are examined. It is found that fragments with fluxes of about 10 to the 21st Mx preserve their identity and survive until the sunspot decays. It is shown that each fragment has a precise location in the ensemble and that the incomplete coalescence of fragments leaves interstices which are penetrated by hot gas (umbral fine structures).