A study of dizygotic twinning in fetal pigs. The fusion of chorions may in the pig as in cattle be followed by the establishment of a common circulation between two individuals. When the individuals are different-sexed, modification of the 9 sex equipment results, and a form of inter-sexuality is produced resembling the bovine free-martin. Three cases of such intergrades are described, the foetuses measuring 4.5 cm., 6 cm., and 9 cm., respectively. An additional case in which the twins were 2 males, one exhibiting several abnormalities of the reproductive system, is also given in detail. In each case the arrangement of the fetal membranes and the histological structure of the reproductive organs are fully described.