Suina Equidae placenta diffuse placenta UBERON:0036151 horses, pigs Almost the entire surface of the allantochorion is involved in formation of the placenta A placenta in which the chorionic sac meets the uterine endometrium over its entire surface. The villi of the chorion are distributed evenly throughout the surface of the chorion, and they extend into processes in the uterine endometrium.