High resolution white light and Hα observations carried out at the Observatorio del Teide (Tenerife) witli the 40 cm Vacuum Telescope of the Kiepenheuer Institute (FRG), allow us to study the processes taking place during the development of 15 large active regions (flux larger than 5x1021Mx). The behaviour of the magnetic fragments which coalesce to form the mature spots, inform us about the nature of the subphotospheric flux rope. The following are the main findings;
(i) Fragments with typically 1021Mx preserve their identity and even survive to the decay of the sunspots.
(ii) Each fragment has a very precise location in the whole ensemble.
(iii) The incomplete coalescense of the fragments leaves interstices which are penetrated by hot gas. Tfiis gives a natural explanation of the umbral fine structures (ie. umbral dots and light bridges).