If the total mass is rho d tau then what is the rate of change of mass per unit time the rate of change of mass per unit time will be equal to del by del t integral of tau rho d tau. That is nothing but integral over tau because tau is a continuous fluid media, so we can always call it del rho by del t d tau. Now, from conservation of mass, hence as I have said that mass can be neither from continuity equation which that is the conservation of mass which says that mass can be neither created nor be destroyed.