Macroscopic appearance of the pigeon crop sac. The non-'lactating' crop (A) has a completely different appearance to that of the 'lactating' crop (B). The lactating crop is more than twice the size of the non-'lactating' crop, with a thickened wall and two very obvious lateral lobes. When the 'lactating' crop is opened the pigeon 'milk' is seen as a bed of close-packed discrete rice-shaped pellets that are closely associated with the mucosal surface of the tissue (C). In contrast, the non-'lactating' crop is undifferentiated with minimal surrounding vasculature.