The mating of the crabs is described. The incubation period of the macrolecithal egg of the freshwater crab, Potamon edulis (Latr.) is 46 ± 1 days long. One of the main features of the development of the egg is its extreme abbreviation. In addition to the nauplius, metanauplius, and zoea, the megalopa stage is also imprisoned in the egg. The extent to which direct development is carried in Potamon is unique among the Brachyura. This is considered to have been a pre-adaptation to the colonization of a freshwater habitat by the Potamonidae.