Complex sentence: In this way it had collected tissue samples of some 2,600,000 people from surgical and autopsy material, mostly in the form of dice-sized pieces of tissue fixed in formalin and embedded in wax blocks of paraffin.
Split into simpler sentences: In this way it has collected tissue samples of some 2,600,000 people from surgical and autopsy material. They mostly take the form of dice-sized pieces of tissue fixed in formalin and embedded in wax blocks of paraffin.