period at sufficiently high redshifts ($z\gtrsim20$), the hyperfine line should be seen in absorption against the CMB, with relative brightness temperatures of up to $|\delta
T_b|\lesssim100$ mK. This is because the IGM initially cools more rapidly than the CMB following recombination [@1994ApJ...427...25S; @1997ApJ...475..429M]. During this period, fluctuations in the differential brightness temperature of the redshifted 21 cm background should track the underlying baryonic matter density perturbations . Eventually, however, the models indicate that the radiation from the first generations of luminous sources will elevate the spin temperature of neutral hydrogen in the IGM above the CMB temperature and the redshifted 21 cm line should be detected in emission with relative brightness temperatures up to the expected maximum values (of order $25$ mK). Finally, during the reionization epoch, the neutral hydrogen becomes ionized, leaving little or no gas to produce the emission, and the apparent differential brightness temperature of the redshifted 21 cm line falls to zero as reionization progresses. As the gas is ionized, a unique pattern should be imprinted in the redshifted 21 cm signal that reflects the processes responsible for the ionizing photons and that evolves with redshift as reionization progresses [@1997ApJ...475..429M; @2000ApJ...528..597T; @2003ApJ...596....1C; @2004ApJ...608..622Z; @2004ApJ...613...16F]. The details 